cmd/go: remove cross compiling restriction on cgo
cmd/go: remove cross compiling restriction on cgo

A previous CL added support for cross compiling with cgo, but
missed the GOOS check in cmd/go. Remove it.

Update issue 4714

LGTM=iant
R=iant
CC=golang-codereviews
https://codereview.appspot.com/57210046

Committer: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang