runtime: replace XOR AX, AX with MOV $0, AX
runtime: replace XOR AX, AX with MOV $0, AX

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/5985048
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go