Make "cpu_relax()" imply a barrier, since that's how it is

used.

This fixes a lockup in synchronize_irq() on x86.

include/asm-i386/processor.h

@@ -455,7 +455,7 @@ struct microcode {
 /* REP NOP (PAUSE) is a good thing to insert into busy-wait loops. */
 static inline void rep_nop(void)
 {
-	__asm__ __volatile__("rep;nop");
+	__asm__ __volatile__("rep;nop": : :"memory");
 }
 
 #define cpu_relax()	rep_nop()