Add new atomic operations and use them in two places.

runtime/vm/atomic_macos.h

Index: runtime/vm/atomic_macos.h
diff --git a/runtime/vm/atomic_macos.h b/runtime/vm/atomic_macos.h
index 479ca30b35394bc25c799efacbf1293377297839..6a30e83b0f645ea0121d63eda85d9f6f917a66bd 100644
--- a/runtime/vm/atomic_macos.h
+++ b/runtime/vm/atomic_macos.h
@@ -15,6 +15,15 @@
 
 namespace dart {
 
+template<typename T> T AtomicOperations::LoadAcquire(T* ptr) {
+  return __atomic_load_n(ptr, __ATOMIC_ACQUIRE);
+}
+
+
+template<typename T> void AtomicOperations::StoreRelease(T* ptr, T val) {
+  __atomic_store_n(ptr, val, __ATOMIC_RELEASE);
+}
+
 
 inline uintptr_t AtomicOperations::FetchAndIncrement(uintptr_t* p) {
   return __sync_fetch_and_add(p, 1);