[sanitizer] Fix build error with current LoongArch Clang

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.

Differential Revision: https://reviews.llvm.org/D137145