[vm] Implement more efficient CAS in simarm/simarm64 modes

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 1ac693ab37a5b68f8af66a55e025dbfbb7432d2c..b60dfed64a8033c30b411709932b4ad3213ede8f 100644
--- a/runtime/vm/atomic_macos.h
+++ b/runtime/vm/atomic_macos.h
@@ -43,7 +43,6 @@ inline void AtomicOperations::DecrementBy(intptr_t* p, intptr_t value) {
   __sync_fetch_and_sub(p, value);
 }
 
-#if !defined(USING_SIMULATOR_ATOMICS)
 inline uword AtomicOperations::CompareAndSwapWord(uword* ptr,
                                                   uword old_value,
                                                   uword new_value) {
@@ -55,7 +54,6 @@ inline uint32_t AtomicOperations::CompareAndSwapUint32(uint32_t* ptr,
                                                        uint32_t new_value) {
   return __sync_val_compare_and_swap(ptr, old_value, new_value);
 }
-#endif  // !defined(USING_SIMULATOR_ATOMICS)
 
 }  // namespace dart