Fixes#5537.
To avoid `go install -v race std` replacing cmd/cgo with a race enabled version and another package trying to build a cgo enabled package, always build cmd/cgo race enabled before doing the rest of the build.
R=remyoudompheng, rsc, dvyukov, minux.ma
CC=golang-dev
https://golang.org/cl/14071044
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://golang.org/cl/7179052
