cmd/gc: fix print format

Fixes 386 build.

TBR=r
CC=golang-codereviews
https://codereview.appspot.com/149620043
cmd/5c, cmd/6c, cmd/8c: make failure to optimize fatal

LGTM=bradfitz, dave, r
R=r, bradfitz, dave
CC=golang-codereviews
https://codereview.appspot.com/152250044