[builtins][LoongArch] Port __clear_cache to LoongArch Linux

[tangyouling]

Summary

There are two failures in the current builtins,
Failed Tests (2):

Builtins-loongarch64-linux :: clear_cache_test.c
Builtins-loongarch64-linux :: enable_execute_stack_test.c

It is caused by __clear_cache not being implemented and triggering abort.

"__clear_cache" is implemented in the same way as "clear_cache" in gcc (
in gcc/config/loongarch/loongarch.md)

Depends on D136338

[MaskRay]

#elif defined(__loongarch__)

should be fine if other ELF operating systems decide to add a loongson port.

[SixWeining]

LGTM. Let's target to Linux firstly and support other systems when necessary. :)

compiler-rt/lib/builtins/clear_cache.c

#define __ARM_NR_cacheflush 0x0f0002
   const register int syscall_nr __asm("r7") = __ARM_NR_cacheflush;
   __asm __volatile("svc 0x0"
                    : "=r"(start_reg)
                    : "r"(syscall_nr), "r"(start_reg), "r"(end_reg), "r"(flags));
   assert(start_reg == 0 && "Cache flush syscall failed.");
 #else
   compilerrt_abort();
 #endif
+#elif defined(__linux__) && defined(__loongarch__)
+  __asm__ volatile("ibar 0");
 #elif defined(__linux__) && defined(__mips__)
   const uintptr_t start_int = (uintptr_t)start;
   const uintptr_t end_int = (uintptr_t)end;
   syscall(__NR_cacheflush, start, (end_int - start_int), BCACHE);
 #elif defined(__mips__) && defined(__OpenBSD__)
   cacheflush(start, (uintptr_t)end - (uintptr_t)start, BCACHE);
 #elif defined(__aarch64__) && !defined(__APPLE__)
   uint64_t xstart = (uint64_t)(uintptr_t)start;