Added asm_volatile_memory

Summary: This adds `asm_volatile_memory`, which goes along with the same style used by `asm_volatile_pause`.
This also switches the two places in `RWSpinLock.h` that were using inline assembly for this to use the new functions instead.
Closes #260

Reviewed By: @yfeldblum

Differential Revision: D2283541

Pulled By: @sgolemon

folly/Portability.h

@@ -339,6 +339,14 @@ inline size_t malloc_usable_size(void* ptr) {
 
 namespace folly {
 
+inline void asm_volatile_memory() {
+#if defined(__clang__) || defined(__GNUC__)
+  asm volatile("" : : : "memory");
+#elif defined(_MSC_VER)
+  ::_ReadWriteBarrier();
+#endif
+}
+
 inline void asm_volatile_pause() {
 #if defined(_MSC_VER) && (defined(_M_IX86) || defined(_M_X64))
   ::_mm_pause();

folly/RWSpinLock.h

@@ -525,13 +525,13 @@ class RWTicketSpinLockT : boost::noncopyable {
  private: // Some x64-specific utilities for atomic access to ticket.
   template<class T> static T load_acquire(T* addr) {
     T t = *addr; // acquire barrier
-    asm volatile("" : : : "memory");
+    asm_volatile_memory();
     return t;
   }
 
   template<class T>
   static void store_release(T* addr, T v) {
-    asm volatile("" : : : "memory");
+    asm_volatile_memory();
     *addr = v; // release barrier
   }