cmd/gc: enforce register-width alignment for stack sizes.
cmd/gc: enforce register-width alignment for stack sizes.

This fixes the following amd64p32 issue:
    pkg/time/format.go:724: internal compiler error: twobitwalktype1: invalid initial alignment, Time

caused by the pointer zone ending on a 32-bit-aligned boundary.

LGTM=rsc
R=rsc, dave
CC=golang-codereviews
https://codereview.appspot.com/72270046

firebitsbr/golang-old,wesolows/golang,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old