cmd/dist: support using cross-compiled std test binaries for slow builders
We want the builders to be able to cross-compile test binaries for a
few of the super slow builders that require either slow hardware or
slow full CPU emulation.
Updates golang/go#31217
Change-Id: I8d33b18efaf788f6f131354b2917ac9738ca975e
Reviewed-on: https://go-review.googlesource.com/c/go/+/178399
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Ian Lance Taylor <iant@golang.org>