src: add race.bash

Add race.bash so anyone with suitable hardware can run a race detector build. race.bash can be called from the dashboard builder by passing -cmd="race.bash".

Original source for race.bash is here, http://code.google.com/p/go-wiki/wiki/DashboardBuilders

TODO: add race.bat for windows/amd64

R=dvyukov, minux.ma, adg, rsc
CC=fullung, golang-dev
https://codereview.appspot.com/7179052