VPERMXOR is missing from the Go assembler for ppc64. It has the
same format as VPERM. It was requested by an external user so
they could write an optimized algorithm in asm.

Change-Id: Icf4c682f7f46716ccae64e6ae3d62e8cec67f6c1
Reviewed-on: https://go-review.googlesource.com/c/151578
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>