Workaround for warning caused by clang pretending to be cl

Summary: Workaround for warning caused by clang pretending to be cl

Differential Revision: D19426072

fbshipit-source-id: ba59913fe9b6c27f32db06f341429952fc4312b5

folly/portability/Asm.h

@@ -26,7 +26,7 @@
 
 namespace folly {
 inline void asm_volatile_memory() {
-#if defined(__GNUC__)
+#if defined(__GNUC__) || defined(__clang__)
   asm volatile("" : : : "memory");
 #elif defined(_MSC_VER)
   ::_ReadWriteBarrier();