cmd/coordinator: support post-submit cross-compiled builds

This adds a boolean to flag a cross compiler config to always be
used for every build, not just when in staging or if it's a try-bot.

It's turned on for the arm5spacemonkey build machines since they take
about 27 minutes to run make.bash.

Fixes golang/go#20118

Change-Id: I13a398d1645a2b5f0c87c5fffc7933922ac1365e
Reviewed-on: https://go-review.googlesource.com/41755
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
1 file changed