quietgcc: fix typo, respect $TMPDIR
quietgcc: fix typo, respect $TMPDIR

R=golang-dev, bradfitz
CC=golang-dev
http://codereview.appspot.com/4529087
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go