cmd/gc: support GNU Bison 2.7 in bisonerrors

R=golang-dev, minux.ma, rsc
CC=golang-dev
https://codereview.appspot.com/7235064
cmd/go: fix build -n for cgo enabled packages

R=golang-dev, minux.ma, rsc
CC=golang-dev
https://codereview.appspot.com/7095067
net/http: fix race

Fixes issue 4724

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/7225071