gomobile init should force download the NDK or the OpenAL package
if -u is set. -x should not print `curl` lines if cached archives
are being used.
Change-Id: I82ceb28540c22a0579ba9969bcf11477f1da81fb
Reviewed-on: https://go-review.googlesource.com/12701
Reviewed-by: David Crawshaw <crawshaw@golang.org>