runtime: remove unnecessary sync from publicationBarrier on s390x
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 <19411bdeea90f848b8e02e9a1c63a7ce7cc699ea@ibm.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Michael Munday <19411bdeea90f848b8e02e9a1c63a7ce7cc699ea@ibm.com>

Cofyc/go,akutz/go,nawawi/go,CAFxX/go,nawawi/go,nawawi/go,codestation/go,nawawi/go,Cofyc/go,CAFxX/go,akutz/go,golang/go,golang/go,akutz/go,cloudflare/go,nawawi/go,akutz/go,CAFxX/go,nawawi/go,golang/go,josharian/go,cloudflare/go,golang/go,cloudflare/go,josharian/go,CAFxX/go,Cofyc/go,Cofyc/go,Cofyc/go,golang/go,Cofyc/go,cloudflare/go,josharian/go,CAFxX/go,codestation/go,codestation/go,akutz/go,josharian/go,christopher-henderson/Go,cloudflare/go,josharian/go,akutz/go,codestation/go,golang/go,josharian/go,nawawi/go,golang/go,Cofyc/go,cloudflare/go,golang/go,CAFxX/go,CAFxX/go,nawawi/go,codestation/go,cloudflare/go,CAFxX/go,akutz/go,josharian/go,akutz/go,codestation/go,Cofyc/go,codestation/go,josharian/go,cloudflare/go,codestation/go