make WMB / MB safer on x86-64

make it so that

if (foo)
	RMB;
else
	MB;

is always done correctly and without syntax surprises

common_x86_64.h

@@ -61,11 +61,11 @@
 */
 
 #ifdef __GNUC__
-#define MB __asm__ __volatile__("": : :"memory")
-#define WMB __asm__ __volatile__("": : :"memory")
+#define MB do { __asm__ __volatile__("": : :"memory"); } while (0)
+#define WMB do { __asm__ __volatile__("": : :"memory"); } while (0)
 #else
-#define MB
-#define WMB
+#define MB do {} while (0)
+#define WMB do {} while (0)
 #endif
 
 static void __inline blas_lock(volatile BLASULONG *address){