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/36832Reviewed-by: Keith Randall <khr@golang.org>