[tsan] Fix pthread_once interceptor for OS X

[kubamracek]

Summary

TSan has a re-implementation of pthread_once in its interceptor, which assumes that the pthread_once_t *once_control pointer is actually pointing to a "storage" which is zero-initialized and used for the atomic operations. However, that's not true on OS X, where pthread_once_t is a structure, that contains a header (with a magic value) and the actual storage follows after that. This patch skips the header to make the interceptor work on OS X.

compiler-rt/trunk/lib/tsan/rtl/tsan_interceptors.cc

TSAN_INTERCEPTOR(int, pthread_barrier_wait, void *b) {
   return res;
 }
 #endif
 
 TSAN_INTERCEPTOR(int, pthread_once, void *o, void (*f)()) {
   SCOPED_INTERCEPTOR_RAW(pthread_once, o, f);
   if (o == 0 || f == 0)
     return EINVAL;
-  atomic_uint32_t *a = static_cast<atomic_uint32_t*>(o);
+  atomic_uint32_t *a;
+  if (!SANITIZER_MAC)
+    a = static_cast<atomic_uint32_t*>(o);
+  else  // On OS X, pthread_once_t has a header with a long-sized signature.
+    a = static_cast<atomic_uint32_t*>((void *)((char *)o + sizeof(long)));
   u32 v = atomic_load(a, memory_order_acquire);
   if (v == 0 && atomic_compare_exchange_strong(a, &v, 1,
                                                memory_order_relaxed)) {
     (*f)();
     if (!thr->in_ignored_lib)
       Release(thr, pc, (uptr)o);
     atomic_store(a, 2, memory_order_release);
   } else {