cmd/gc: reject methods on non-locals, even if symbol exists

Fixes issue 5089.

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/7767044
cmd/gc: add OCHECKNOTNIL to race walker (fix build)

TBR=dvyukov
CC=golang-dev
https://codereview.appspot.com/7635046