Fairly straightforward. A couple of unusual addressing tricks.
Also added the ability to write R(10) to mean R10. PPC64 uses
this for a couple of large register spaces. It appears for ARM now
as well, since I saw some uses of that before, although I rewrote
them in our source. I could put it in for 386 and amd64 but it's
not worth it.

Change-Id: I3ffd7ffa62d511b95b92c3c75b9f1d621f5393b6
Reviewed-on: https://go-review.googlesource.com/5282Reviewed-by: Russ Cox <rsc@golang.org>