Bug 1210456 - Add atomic *SafeWhenRacy() for ARM64. r=lth

These methods landed with 1084248 for other architectures. A real
implementation is still needed, but this allows the code to build.

js/src/jit/arm64/AtomicOperations-arm64.h

@@ -104,6 +104,34 @@ js::jit::AtomicOperations::fetchXorSeqCst(T* addr, T val)
     return __atomic_fetch_xor(addr, val, __ATOMIC_SEQ_CST);
 }
 
+template <typename T>
+inline T
+js::jit::AtomicOperations::loadSafeWhenRacy(T* addr)
+{
+    return *addr; // FIXME (1208663): not yet safe
+}
+
+template <typename T>
+inline void
+js::jit::AtomicOperations::storeSafeWhenRacy(T* addr, T val)
+{
+    *addr = val; // FIXME (1208663): not yet safe
+}
+
+inline void
+js::jit::AtomicOperations::memcpySafeWhenRacy(void* dest, const void* src,
+                                              size_t nbytes)
+{
+    memcpy(dest, src, nbytes); // FIXME (1208663): not yet safe
+}
+
+inline void
+js::jit::AtomicOperations::memmoveSafeWhenRacy(void* dest, const void* src,
+                                               size_t nbytes)
+{
+    memmove(dest, src, nbytes); // FIXME (1208663): not yet safe
+}
+
 template<size_t nbytes>
 inline void
 js::jit::RegionLock::acquire(void* addr)