Use atomic variable for Runtime ref_count_.

Change-Id: Ic4d0ad9ff93d0cc52cfe2df006ee3436d5960b07

runtime/hsa-runtime/core/inc/runtime.h

@@ -510,7 +510,7 @@ class Runtime {
   void* vm_fault_handler_user_data_;
 
   // Holds reference count to runtime object.
-  volatile uint32_t ref_count_;
+  std::atomic<uint32_t> ref_count_;
 
   // Track environment variables.
   Flag flag_;