cmd/go: diagnose un-bootstrapped runtime
cmd/go: diagnose un-bootstrapped runtime

Fixes issue 4665.

R=golang-dev, minux.ma
CC=golang-dev
https://codereview.appspot.com/7132057
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go