cmd/go: fix data race during build Fixes issue 2695.
cmd/go: fix data race during build
Fixes issue 2695.

R=golang-dev, mpimenov, minux.ma, rsc
CC=golang-dev
http://codereview.appspot.com/5545052
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go