[release-branch.go1.3] cmd/8g: fix build

««« CL 129720043 / 0449858880be
cmd/8g: fix build

Fixes issue 8510.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/129720043
»»»

TBR=rsc
CC=golang-codereviews
https://codereview.appspot.com/129760043