I looked into enabling this but it's not incredibly straightforward.
We can't just flip a bit on misc-compile-* builders because misc-compile uses the buildall.bash build script.
Looks like we'd need to write some code in the coordinator for it.
In addition to just running the equivalent of GOOS=os GOARCH=arch go install golang.org/x/subrepo/..., we'd probably also want to enumerate all the packages with tests and, for each, run GOOS=os GOARCH=arch go test -c golang.org/x/subrepo/each/package. (@josharian, where's that bug about not being able to use go test -c with multiple packages? That'd be nice here if we had it. Oh well.)