[sanitizer_common] Don't use syscall(SYS_clone) on Linux/sparc64 (llvm#100534)

```
  SanitizerCommon-Unit :: ./Sanitizer-sparc-Test/SanitizerCommon/StartSubprocessTest
```
and every single test using the `llvm-symbolizer` `FAIL` on
Linux/sparc64 in a very weird way: when using `StartSubprocess`, there's
a call to `internal_fork`, but we never reach `internal_execve`.
`internal_fork` is implemented using `syscall(SYS_clone)`. The calling
convention of that syscall already varies considerably between targets,
but as documented in `clone(2)`, SPARC again is widely different.
Instead of trying to match `glibc` here, this patch just calls `__fork`.

Tested on `sparc64-unknown-linux-gnu` and `x86_64-pc-linux-gnu`.

(cherry picked from commit 1c53b90)

Author: rorth

compiler-rt/lib/sanitizer_common/sanitizer_linux.cpp

@@ -826,10 +826,16 @@ uptr internal_sigaltstack(const void *ss, void *oss) {
   return internal_syscall(SYSCALL(sigaltstack), (uptr)ss, (uptr)oss);
 }
 
+extern "C" pid_t __fork(void);
+
 int internal_fork() {
 #    if SANITIZER_LINUX
 #      if SANITIZER_S390
   return internal_syscall(SYSCALL(clone), 0, SIGCHLD);
+#      elif SANITIZER_SPARC
+  // The clone syscall interface on SPARC differs massively from the rest,
+  // so fall back to __fork.
+  return __fork();
 #      else
   return internal_syscall(SYSCALL(clone), SIGCHLD, 0);
 #      endif