runtime: remove unnecessary sync from publicationBarrier on s390x

Memory accesses on z are at least as ordered as they are on AMD64.

Change-Id: Ia515430e571ebd07e9314de05c54dc992ab76b95
Reviewed-on: https://go-review.googlesource.com/74010
Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Munday <mike.munday@ibm.com>

src/runtime/asm_s390x.s

@@ -915,7 +915,8 @@ TEXT runtime·sigreturn(SB),NOSPLIT,$0-0
 	RET
 
 TEXT ·publicationBarrier(SB),NOSPLIT|NOFRAME,$0-0
-	SYNC
+        // Stores are already ordered on s390x, so this is just a
+        // compile barrier.
 	RET
 
 TEXT runtime·cmpstring(SB),NOSPLIT|NOFRAME,$0-40