cmd/gc: racewalk: fix instrumentation of structs + do not instrument go.itab.*
cmd/gc: racewalk: fix instrumentation of structs
+ do not instrument go.itab.*

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/6819106
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go