cmd/go: allow underscores in tool name         Otherwise we can't invoke go_bootstrap directly.
cmd/go: allow underscores in tool name
        Otherwise we can't invoke go_bootstrap directly.

R=golang-dev, r
CC=golang-dev
http://codereview.appspot.com/5900061

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang