cmd/go: diagnose un-bootstrapped runtime

Fixes issue 4665.

R=golang-dev, minux.ma
CC=golang-dev
https://codereview.appspot.com/7132057
cmd/gc: allow registerization of temporaries created by inlining.

Names beginning with a dot are ignored by optimizers.

R=rsc, lvd, golang-dev, dave
CC=golang-dev
https://codereview.appspot.com/7098049