This is preliminary work to unifying the
unix amd64 assembly implementations,
which is preliminary work to making the
assembly vet-friendly.

Updates #11041

Change-Id: Ic64985124f8fb86cc08898be2ec7fca972ced4ca
Reviewed-on: https://go-review.googlesource.com/25113
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>