powerpc: add gmb barrier

CVE-2017-5753
CVE-2017-5715
Signed-off-by: Andy Whitcroft <apw@canonical.com>
(backported from commit 1d40e84780e12578489810065695f5e72c7d1b89)
Signed-off-by: Andy Whitcroft <apw@canonical.com>

arch/powerpc/include/asm/barrier.h

@@ -36,6 +36,8 @@
 
 #define smp_store_mb(var, value) do { WRITE_ONCE(var, value); smp_mb(); } while (0)
 
+#define gmb()   asm volatile("ori 31,31,0")
+
 #ifdef __SUBARCH_HAS_LWSYNC
 #    define SMPWMB      LWSYNC
 #else