tsan: fix fork syscall test

[dvyukov]

Summary

Arm64 builders failed with:
error: use of undeclared identifier 'SYS_fork'
https://lab.llvm.org/buildbot/#/builders/7/builds/2575

Indeed, not all arches have fork syscall.
Implement fork via clone on these arches.

compiler-rt/test/tsan/Linux/fork_syscall.cpp

 static void *incrementer(void *p) {
   for (;;)
     __sync_fetch_and_add(&counter, 1);
   return 0;
 }
 
 int myfork() {
   __sanitizer_syscall_pre_fork();
+#ifdef SYS_fork
   int res = syscall(SYS_fork);
+#else
+  int res = syscall(SYS_clone, SIGCHLD, 0);
+#endif
   __sanitizer_syscall_post_fork(res);
   return res;
 }
 
 int main() {
   barrier_init(&barrier, 2);
   pthread_t th1;
   pthread_create(&th1, 0, incrementer, 0);