This disables the s390x assembly. It will be re-enabled when #20215
is resolved on s390x.

Change-Id: I789eca2dd478004956107359fae98ed012f04abb
Reviewed-on: https://go-review.googlesource.com/62292
Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>