race.bash, race.bat: build a race enabled cmd/cgo before trying to use it

Fixes issue 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://codereview.appspot.com/14071044