cmd/gc: fix typos in clearslim.
cmd/gc: fix typos in clearslim.

Fixes build failure.

R=golang-dev, bradfitz, dave
CC=golang-dev
http://codereview.appspot.com/6847043

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