This adds support for ppc64 instructions vmrgow and vmrgew which
are needed for an improved implementation of chacha20.

Change-Id: I967a2de54236bcc573a99f7e2b222d5a8bb29e03
Reviewed-on: https://go-review.googlesource.com/c/go/+/192117
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Carlos Eduardo Seo <cseo@linux.vnet.ibm.com>