ppc64: add, from github.com/minux/power64

This is a direct copy of github.com/minux/power64,
which in turn was forked from rsc.io/power64,
which was using Mercurial and is gone.
The code in those places was reviewed via code review
and is by Minux and me under the usual Go CLA.

I've done a global search and replace of power64 to ppc64
and checked that everything still builds. Any further fixes
will be in followup CLs.

(The arch subrepo did not exist when this code was written.)

Change-Id: I80ea16ca689c9fc51a7501c3492099f19aa30873
Reviewed-on: https://go-review.googlesource.com/30932
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
16 files changed