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 #4714

LGTM=iant
R=iant
CC=golang-codereviews
https://golang.org/cl/57210046

src/cmd/go/build.go

@@ -2068,10 +2068,6 @@ var (
 )
 
 func (b *builder) cgo(p *Package, cgoExe, obj string, gccfiles, gxxfiles, mfiles []string) (outGo, outObj []string, err error) {
-	if goos != toolGOOS {
-		return nil, nil, errors.New("cannot use cgo when compiling for a different operating system")
-	}
-
 	cgoCPPFLAGS := stringList(envList("CGO_CPPFLAGS"), p.CgoCPPFLAGS)
 	cgoCFLAGS := stringList(envList("CGO_CFLAGS"), p.CgoCFLAGS)
 	cgoCXXFLAGS := stringList(envList("CGO_CXXFLAGS"), p.CgoCXXFLAGS)