Add new atomic operations and use them in two places.

runtime/vm/atomic.h

Index: runtime/vm/atomic.h
diff --git a/runtime/vm/atomic.h b/runtime/vm/atomic.h
index b6e5f280001ae5f61fa3c8e10d339f9a91de478d..edf1990fe13585dfac659f90a82c272b6e4bd2d2 100644
--- a/runtime/vm/atomic.h
+++ b/runtime/vm/atomic.h
@@ -60,6 +60,10 @@ class AtomicOperations : public AllStatic {
   static intptr_t LoadRelaxedIntPtr(intptr_t* ptr) {
     return *static_cast<volatile intptr_t*>(ptr);
   }
+
+  template<typename T> static T LoadAcquire(T* ptr);
+
+  template<typename T> static void StoreRelease(T* ptr, T val);
 };