[sanitizer] Fix build error with current LoongArch Clang

[tangyouling]

Summary

Fix the following build failures:

In file included from /home/loongson/llvm-work/llvm-project/compiler-rt/lib/sanitizer_common/sanit>
llvm-project/compiler-rt/lib/sanitizer_common/sanitizer_syscall_linux_loongarch64.inc:27:23: error>
  register u64 a7 asm("a7") = nr;
                      ^
llvm-project/compiler-rt/lib/sanitizer_common/sanitizer_syscall_linux_loongarch64.inc:28:23: error>
  register u64 a0 asm("a0");                      ^

The non-prefix $ syntax in inline assembly is not supported in clang
yet (it is supported by gcc), add prefix $ to solve the problem.

[SixWeining]

LGTM based on the background in D136436.

[xry111]

LGTM. But I'd wait for the other's opinion before giving a formal approve.

[xen0n]

I can confirm this doesn't affect functionality and is only for toolchain compatibility purposes.

compiler-rt/lib/sanitizer_common/sanitizer_syscall_linux_loongarch64.inc

 //  (http://man7.org/linux/man-pages/man2/syscall.2.html) results are return in
 //  a0 and a1 (http://man7.org/linux/man-pages/man2/syscall.2.html) arguments
 //  are passed in: a0-a7 (confirmed by inspecting glibc sources).
 #define SYSCALL(name) __NR_##name
 
 #define INTERNAL_SYSCALL_CLOBBERS "memory"
 
 static uptr __internal_syscall(u64 nr) {
-  register u64 a7 asm("a7") = nr;
-  register u64 a0 asm("a0");
+  register u64 a7 asm("$a7") = nr;
+  register u64 a0 asm("$a0");
   __asm__ volatile("syscall 0\n\t"
                    : "=r"(a0)
                    : "r"(a7)
                    : INTERNAL_SYSCALL_CLOBBERS);
   return a0;
 }
 #define __internal_syscall0(n) (__internal_syscall)(n)
 
 static uptr __internal_syscall(u64 nr, u64 arg1) {
-  register u64 a7 asm("a7") = nr;
-  register u64 a0 asm("a0") = arg1;
+  register u64 a7 asm("$a7") = nr;
+  register u64 a0 asm("$a0") = arg1;
   __asm__ volatile("syscall 0\n\t"
                    : "+r"(a0)
                    : "r"(a7)
                    : INTERNAL_SYSCALL_CLOBBERS);
   return a0;
 }
 #define __internal_syscall1(n, a1) (__internal_syscall)(n, (u64)(a1))
 
 static uptr __internal_syscall(u64 nr, u64 arg1, long arg2) {
-  register u64 a7 asm("a7") = nr;
-  register u64 a0 asm("a0") = arg1;
-  register u64 a1 asm("a1") = arg2;
+  register u64 a7 asm("$a7") = nr;
+  register u64 a0 asm("$a0") = arg1;
+  register u64 a1 asm("$a1") = arg2;
   __asm__ volatile("syscall 0\n\t"
                    : "+r"(a0)
                    : "r"(a7), "r"(a1)
                    : INTERNAL_SYSCALL_CLOBBERS);
   return a0;
 }
 #define __internal_syscall2(n, a1, a2) \
   (__internal_syscall)(n, (u64)(a1), (long)(a2))
 
 static uptr __internal_syscall(u64 nr, u64 arg1, long arg2, long arg3) {
-  register u64 a7 asm("a7") = nr;
-  register u64 a0 asm("a0") = arg1;
-  register u64 a1 asm("a1") = arg2;
-  register u64 a2 asm("a2") = arg3;
+  register u64 a7 asm("$a7") = nr;
+  register u64 a0 asm("$a0") = arg1;
+  register u64 a1 asm("$a1") = arg2;
+  register u64 a2 asm("$a2") = arg3;
   __asm__ volatile("syscall 0\n\t"
                    : "+r"(a0)
                    : "r"(a7), "r"(a1), "r"(a2)
                    : INTERNAL_SYSCALL_CLOBBERS);
   return a0;
 }
 #define __internal_syscall3(n, a1, a2, a3) \
   (__internal_syscall)(n, (u64)(a1), (long)(a2), (long)(a3))
 
 static uptr __internal_syscall(u64 nr, u64 arg1, long arg2, long arg3,
                                u64 arg4) {
-  register u64 a7 asm("a7") = nr;
-  register u64 a0 asm("a0") = arg1;
-  register u64 a1 asm("a1") = arg2;
-  register u64 a2 asm("a2") = arg3;
-  register u64 a3 asm("a3") = arg4;
+  register u64 a7 asm("$a7") = nr;
+  register u64 a0 asm("$a0") = arg1;
+  register u64 a1 asm("$a1") = arg2;
+  register u64 a2 asm("$a2") = arg3;
+  register u64 a3 asm("$a3") = arg4;
   __asm__ volatile("syscall 0\n\t"
                    : "+r"(a0)
                    : "r"(a7), "r"(a1), "r"(a2), "r"(a3)
                    : INTERNAL_SYSCALL_CLOBBERS);
   return a0;
 }
 #define __internal_syscall4(n, a1, a2, a3, a4) \
   (__internal_syscall)(n, (u64)(a1), (long)(a2), (long)(a3), (long)(a4))
 
 static uptr __internal_syscall(u64 nr, u64 arg1, long arg2, long arg3, u64 arg4,
                                long arg5) {
-  register u64 a7 asm("a7") = nr;
-  register u64 a0 asm("a0") = arg1;
-  register u64 a1 asm("a1") = arg2;
-  register u64 a2 asm("a2") = arg3;
-  register u64 a3 asm("a3") = arg4;
-  register u64 a4 asm("a4") = arg5;
+  register u64 a7 asm("$a7") = nr;
+  register u64 a0 asm("$a0") = arg1;
+  register u64 a1 asm("$a1") = arg2;
+  register u64 a2 asm("$a2") = arg3;
+  register u64 a3 asm("$a3") = arg4;
+  register u64 a4 asm("$a4") = arg5;
   __asm__ volatile("syscall 0\n\t"
                    : "+r"(a0)
                    : "r"(a7), "r"(a1), "r"(a2), "r"(a3), "r"(a4)
                    : INTERNAL_SYSCALL_CLOBBERS);
   return a0;
 }
 #define __internal_syscall5(n, a1, a2, a3, a4, a5)                       \
   (__internal_syscall)(n, (u64)(a1), (long)(a2), (long)(a3), (long)(a4), \
                        (u64)(a5))
 
 static uptr __internal_syscall(u64 nr, u64 arg1, long arg2, long arg3, u64 arg4,
                                long arg5, long arg6) {
-  register u64 a7 asm("a7") = nr;
-  register u64 a0 asm("a0") = arg1;
-  register u64 a1 asm("a1") = arg2;
-  register u64 a2 asm("a2") = arg3;
-  register u64 a3 asm("a3") = arg4;
-  register u64 a4 asm("a4") = arg5;
-  register u64 a5 asm("a5") = arg6;
+  register u64 a7 asm("$a7") = nr;
+  register u64 a0 asm("$a0") = arg1;
+  register u64 a1 asm("$a1") = arg2;
+  register u64 a2 asm("$a2") = arg3;
+  register u64 a3 asm("$a3") = arg4;
+  register u64 a4 asm("$a4") = arg5;
+  register u64 a5 asm("$a5") = arg6;
   __asm__ volatile("syscall 0\n\t"
                    : "+r"(a0)
                    : "r"(a7), "r"(a1), "r"(a2), "r"(a3), "r"(a4), "r"(a5)
                    : INTERNAL_SYSCALL_CLOBBERS);
   return a0;
 }
 #define __internal_syscall6(n, a1, a2, a3, a4, a5, a6)                   \
   (__internal_syscall)(n, (u64)(a1), (long)(a2), (long)(a3), (long)(a4), \
                        (u64)(a5), (long)(a6))
 
 static uptr __internal_syscall(u64 nr, u64 arg1, long arg2, long arg3, u64 arg4,
                                long arg5, long arg6, long arg7) {
-  register u64 a7 asm("a7") = nr;
-  register u64 a0 asm("a0") = arg1;
-  register u64 a1 asm("a1") = arg2;
-  register u64 a2 asm("a2") = arg3;
-  register u64 a3 asm("a3") = arg4;
-  register u64 a4 asm("a4") = arg5;
-  register u64 a5 asm("a5") = arg6;
-  register u64 a6 asm("a6") = arg7;
+  register u64 a7 asm("$a7") = nr;
+  register u64 a0 asm("$a0") = arg1;
+  register u64 a1 asm("$a1") = arg2;
+  register u64 a2 asm("$a2") = arg3;
+  register u64 a3 asm("$a3") = arg4;
+  register u64 a4 asm("$a4") = arg5;
+  register u64 a5 asm("$a5") = arg6;
+  register u64 a6 asm("$a6") = arg7;
   __asm__ volatile("syscall 0\n\t"
                    : "+r"(a0)
                    : "r"(a7), "r"(a1), "r"(a2), "r"(a3), "r"(a4), "r"(a5),
                      "r"(a6)
                    : INTERNAL_SYSCALL_CLOBBERS);
   return a0;
 }
 #define __internal_syscall7(n, a1, a2, a3, a4, a5, a6, a7)               \