cmd/gc: mark LOCALS argument as a constant to print correctly
cmd/gc: mark LOCALS argument as a constant to print correctly

Fixes issue 4875.

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/7376049

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