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

runtime/vm/atomic_fuchsia.h

Index: runtime/vm/atomic_fuchsia.h
diff --git a/runtime/vm/atomic_fuchsia.h b/runtime/vm/atomic_fuchsia.h
index fd873ed779fe7d5117a4c505008f71d723f0ce30..2076487911687f4611400c7af928ed2db62e2cfb 100644
--- a/runtime/vm/atomic_fuchsia.h
+++ b/runtime/vm/atomic_fuchsia.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