Auto merge of #981 - RalfJung:atomic, r=RalfJung

also test AcqRel fence

Missing from #979

Author: bors

tests/run-pass/atomic.rs

@@ -69,4 +69,5 @@ fn atomic_fences() {
     fence(SeqCst);
     fence(Release);
     fence(Acquire);
+    fence(AcqRel);
 }