cmd/gc: racewalk: handle OEFACE/OCONVIFACE
cmd/gc: racewalk: handle OEFACE/OCONVIFACE

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/6821096

wesolows/golang,wesolows/golang,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old