dashboard: convert plan9-arm, plan9-amd64-9front, nacl-arm to buildlets
dashboard: convert plan9-arm, plan9-amd64-9front, nacl-arm to buildlets

Updates golang/go#21191

Change-Id: I577dad51d490d7e281ccde9fa7d82bb1bee95bd9
Reviewed-on: https://go-review.googlesource.com/53417
Reviewed-by: David du Colombier <ec8ae1e6620d4bab5e0e7aa2a6ae41aad63e3708@gmail.com>

golang/build,golang/build,golang/build,golang/build