powerpc: Wire up sys_membarrier()

The selftest passes on 64-bit LE & BE, and 32-bit.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
diff --git a/arch/powerpc/include/asm/systbl.h b/arch/powerpc/include/asm/systbl.h
index 4d65499..126d0c4 100644
--- a/arch/powerpc/include/asm/systbl.h
+++ b/arch/powerpc/include/asm/systbl.h
@@ -369,3 +369,4 @@
 COMPAT_SYS(execveat)
 PPC64ONLY(switch_endian)
 SYSCALL_SPU(userfaultfd)
+SYSCALL_SPU(membarrier)