cmd/builder: cross compile darwin/arm
cmd/builder: cross compile darwin/arm

Change-Id: I09ef985a24f9aa1ba341d9d634d45c6dea8d63a5
Reviewed-on: https://go-review.googlesource.com/5051
Reviewed-by: Minux Ma <72d850dbd8858b791995463d8539b5a379577f28@golang.org>

golang/build,proppy/build,golang/build,evandbrown/build,golang/build,evandbrown/build,golang/build,evandbrown/build,proppy/build,proppy/build,proppy/build