folly: avoid using atomic operations android can't handle

Summary:On Android, clang 3.8 + libstdc++ doesn't support the full
range of atomic operations.  This diff fixes up the ones we
ran into when building fb4a.

Reviewed By: mzlee

Differential Revision: D3092352

fb-gh-sync-id: 7fea8513e23425fc37050ad14d82aabaceb00352
shipit-source-id: 7fea8513e23425fc37050ad14d82aabaceb00352

folly/experimental/TLRefCount.h

@@ -76,7 +76,7 @@ class TLRefCount {
 
     assert(state_.load() == State::GLOBAL);
 
-    return --globalCount_;
+    return globalCount_-- - 1;
   }
 
   Int operator*() const {
@@ -132,7 +132,7 @@ class TLRefCount {
       }
 
       collectCount_ = count_;
-      refCount_.globalCount_ += collectCount_;
+      refCount_.globalCount_.fetch_add(collectCount_);
       collectGuard_.reset();
     }