cmd/compile: intrinsify atomics on ppc64x

cmd/compile: intrinsify atomics on ppc64x

This adds the necessary changes so that atomics are treated as
intrinsics on ppc64x.

The implementations of And8 and Or8 require power8 for
both ppc64 and ppc64le.  This is a new requirement
for ppc64.

Fixes #8739

Change-Id: Icb85e2755a49166ee3652668279f6ed5ebbca901
Reviewed-on: https://go-review.googlesource.com/36832
Reviewed-by: Keith Randall <khr@golang.org>