Only use _ReadWriteBarrier on MSVC

It was causing compile errors when cross-compiling using mingw.

No functional change.

src/syzygy/tbprobe.cpp

@@ -146,8 +146,8 @@ static int probe_wdl_table(Position& pos, int *success)
         return 0;
       }
       // Memory barrier to ensure ptr->ready = 1 is not reordered.
-#ifdef _WIN32
-	  _ReadWriteBarrier();
+#ifdef _MSC_VER
+      _ReadWriteBarrier();
 #else
       __asm__ __volatile__ ("" ::: "memory");
 #endif