Use memcpy for Atomic::LoadRelaxed.

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 46d0bec4b612633260f3b69e18c52096ae0a23be..bc2ed6a17a9eceff5ebc03e76e5f028b9446dc02 100644
--- a/runtime/vm/atomic_macos.h
+++ b/runtime/vm/atomic_macos.h
@@ -34,17 +34,6 @@ inline uword AtomicOperations::CompareAndSwapWord(uword* ptr,
 }
 #endif  // !defined(USING_SIMULATOR)
 
-
-inline uword AtomicOperations::LoadRelaxed(uword* ptr) {
-#if defined(__ATOMIC_RELAXED)
-  return __c11_atomic_load(ptr, __ATOMIC_RELAXED);
-#else
-  // TODO(koda): Consider using C++11 <atomic> to avoid the barrier on more
-  // compilers/platforms.
-  return FetchAndAdd(reinterpret_cast<intptr_t*>(ptr), 0);
-#endif
-}
-
 }  // namespace dart
 
 #endif  // VM_ATOMIC_MACOS_H_