cmd/dist: support using cross-compiled std test binaries for slow builders
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 <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

CAFxX/go,CAFxX/go,golang/go,nawawi/go,akutz/go,golang/go,cloudflare/go,akutz/go,golang/go,CAFxX/go,cloudflare/go,akutz/go,cloudflare/go,CAFxX/go,nawawi/go,golang/go,CAFxX/go,cloudflare/go,golang/go,nawawi/go,nawawi/go,golang/go,akutz/go,akutz/go,CAFxX/go,golang/go,cloudflare/go,nawawi/go,nawawi/go,nawawi/go,nawawi/go,cloudflare/go,CAFxX/go,cloudflare/go,golang/go,akutz/go,akutz/go,cloudflare/go,akutz/go,CAFxX/go