[release-branch.go1.5] cmd/compile: do not let regopt use REGTMP on ppc64

ppc64 codegen assumes that it is OK to stomp on r31 at any time, but it is not
excluded from the set of registers that regopt is allowed to use.

Fixes #12597

Change-Id: I29c7655e32abd22f3c21d88427b73e4fca055233
Reviewed-on: https://go-review.googlesource.com/15245


Reviewed-by: Minux Ma <minux@golang.org>
Reviewed-on: https://go-review.googlesource.com/16981


Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>