[libc] Add Convenience syscall Template Function

[abrachet]

Summary

This patch adds a convenience template function so we don't need to cast all types to long manually when calling __llvm_libc::syscall.

[gchatelet]

Yeah that would be nice to separate the formatting [NFC] from the template addition. It's hard spot what changed in the file :)

[sivachandra]

Yes, can you please separate the formatting from the actual change?

libc/config/linux/x86_64/syscall.h.inc

__attribute__((always_inline)) inline long syscall(long __number, long __arg1,
   LIBC_INLINE_ASM("syscall"
                   : "=a"(retcode)
                   : "a"(__number), "D"(__arg1), "S"(__arg2), "d"(__arg3),
                     "r"(r10), "r"(r8), "r"(r9)
                   : SYSCALL_CLOBBER_LIST);
   return retcode;
 }
 
+template <typename... Ts>
+__attribute__((always_inline)) inline long syscall(long __number, Ts... ts) {
+  static_assert(sizeof...(Ts) <= 6, "Too many arguments for syscall");
+  return syscall(__number, (long)ts...);
+}
+
+
 #undef SYSCALL_CLOBBER_LIST
 
 } // namespace __llvm_libc

libc/test/config/linux/x86_64/syscall_test.cpp

TEST(X86_64_SyscallTest, APITest) {
   std::function<long(long, long, long, long, long, long)> f5(
       [](long n, long a1, long a2, long a3, long a4, long a5) {
         return __llvm_libc::syscall(n, a1, a2, a3, a4, a5);
       });
   std::function<long(long, long, long, long, long, long, long)> f6(
       [](long n, long a1, long a2, long a3, long a4, long a5, long a6) {
         return __llvm_libc::syscall(n, a1, a2, a3, a4, a5, a6);
       });
+
+  std::function<long(long, void *)> notLongType(
+      [](long n, void *a1) {
+        return __llvm_libc::syscall(n, a1);
+      });
 }