[POWERPC] Fix rmb() for e500-based machines it

The e500 core generates an illegal instruction exception when it tries
to execute the lwsync instruction, which we currently use for rmb().
This fixes it by using the LWSYNC macro, which turns into a plain sync
on 32-bit machines.
Signed-off-by: Andrew Fleming <afleming@freescale.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>

include/asm-powerpc/system.h

@@ -25,8 +25,8 @@
  *
  * We have to use the sync instructions for mb(), since lwsync doesn't
  * order loads with respect to previous stores.  Lwsync is fine for
- * rmb(), though.  Note that lwsync is interpreted as sync by
- * 32-bit and older 64-bit CPUs.
+ * rmb(), though. Note that rmb() actually uses a sync on 32-bit
+ * architectures.
  *
  * For wmb(), we use sync since wmb is used in drivers to order
  * stores to system memory with respect to writes to the device.
@@ -34,7 +34,7 @@
  * SMP since it is only used to order updates to system memory.
  */
 #define mb()   __asm__ __volatile__ ("sync" : : : "memory")
-#define rmb()  __asm__ __volatile__ ("lwsync" : : : "memory")
+#define rmb()  __asm__ __volatile__ (__stringify(LWSYNC) : : : "memory")
 #define wmb()  __asm__ __volatile__ ("sync" : : : "memory")
 #define read_barrier_depends()  do { } while(0)