dashboard: add s390x cross-compile-only builder and release config

Also modifies cmd/release to wire it up.

Updates golang/go#16443

Change-Id: Ie80a224957f6f662d0aec0fd083e2a57e30436c4
Reviewed-on: https://go-review.googlesource.com/28172
Reviewed-by: Jessica Frazelle <me@jessfraz.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
4 files changed