Adds support for single block encryption using the cipher message
(KM) instruction. KM handles key expansion internally and
therefore it is not done up front when using the assembly
implementation on s390x.

Change-Id: I69954b8ae36d549e1dc40d7acd5a10bedfaaef9c
Reviewed-on: https://go-review.googlesource.com/22194
Run-TryBot: Michael Munday <munday@ca.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bill O'Farrell <billotosyr@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>