Fix build by casting constants and using more compatible intrinsic name.

runtime/vm/atomic_macos.h

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef VM_ATOMIC_MACOS_H_
 #define VM_ATOMIC_MACOS_H_
 
 #if !defined VM_ATOMIC_H_
 #error Do not include atomic_macos.h directly. Use atomic.h instead.
 #endif
@@ skipping 19 matching lines @@
 inline uword AtomicOperations::CompareAndSwapWord(uword* ptr,
                                                   uword old_value,
                                                   uword new_value) {
   return __sync_val_compare_and_swap(ptr, old_value, new_value);
 }
 #endif  // !defined(USING_SIMULATOR)
 
 
 inline uword AtomicOperations::LoadRelaxed(uword* ptr) {
 #if defined(__ATOMIC_RELAXED)
-  return __atomic_load_n(ptr, __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_