gobuilder: permit builders of the form goos-goarch-foo
gobuilder: permit builders of the form goos-goarch-foo

R=dfc
CC=golang-dev
http://codereview.appspot.com/4416044

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