Part of the x86 asm backend cleanup. Passes toolstash-check.

Change-Id: Ib9766689fb7edb07f5553868cf89175501ea1e1b
Reviewed-on: https://go-review.googlesource.com/107055Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>