Use constants directly, instead of loading address to e. g. AX
and using (AX). Shouldn't affect performance, but makes code a bit
nicer.

Change-Id: Ifa138e54d3d2b2f4ad71e4ef4b9368ea79eb30f4
Reviewed-on: https://go-review.googlesource.com/62010Reviewed-by: Adam Langley <agl@golang.org>