Disable PthreadKeyUnregister on iOS and MSVC

Summary: ~PthreadKeyUnregister logic is not safe if we can't guarantee that it has the maximum pririty (i.e. is the last to be run on shutdown).

Reviewed By: ericniebler

Differential Revision: D3517589

fbshipit-source-id: 3340e2e19cf52973ee677288bc4ac6105f3f2543

folly/detail/ThreadLocalDetail.h

@@ -172,10 +172,14 @@ class PthreadKeyUnregister {
   static constexpr size_t kMaxKeys = 1UL << 16;
 
   ~PthreadKeyUnregister() {
+    // If static constructor priorities are not supported then
+    // ~PthreadKeyUnregister logic is not safe.
+#if !defined(__APPLE__) && !defined(_MSC_VER)
     MSLGuard lg(lock_);
     while (size_) {
       pthread_key_delete(keys_[--size_]);
     }
+#endif
   }
 
   static void registerKey(pthread_key_t key) {