qemu-barrier: Fix build failure on PowerPC Mac OS X

qemu-barrier.h tests if macro __powerpc__ is defined, however, the
preprocessor on PowerPC Mac OS X defines only __POWERPC__, not
__powerpc__.  Resolve by testing instead for qemu-provided _ARCH_PPC.
Signed-off-by: NEric Sunshine <sunshine@sunshineco.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

qemu-barrier.h

@@ -14,7 +14,7 @@
  */
 #define smp_wmb()   barrier()
 
-#elif defined(__powerpc__)
+#elif defined(_ARCH_PPC)
 
 /*
  * We use an eieio() for a wmb() on powerpc.  This assumes we don't