alpha: Use mb for rmb

Alpha never have a rmb instruction.
Just use mb for rmb.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>

common_alpha.h

@@ -43,7 +43,7 @@
 
 #define MB  asm("mb")
 #define WMB asm("wmb")
-#define RMB asm("rmb")
+#define RMB asm("mb")
 
 static void __inline blas_lock(unsigned long *address){
 #ifndef __DECC