This lets us mark net's cgo_stub.go as only to be built when cgo is disabled. R=golang-dev, ality, mikioh.mikioh CC=golang-dev https://golang.org/cl/5489100
R=golang-dev, r CC=golang-dev https://golang.org/cl/5484073
Fixes #1009. R=adg, rsc CC=golang-dev https://golang.org/cl/5262041