x/build: trybots should always compile for all platforms on subrepos

[bradfitz]

https://go-review.googlesource.com/c/47290/ broke the build for freebsd-386 which was reported in #21136

@ncw said:

According to https://go-review.googlesource.com/c/47290/ the trybots ran OK so I'm a bit puzzled about this.

We don't run all platforms for trybots since trybots are somewhat expensive, so we choose where to spend our resources. (We run freebsd-amd64 and linux-386 but not freebsd-386, for instance)

But we should at least run misc-compile (or equivalent) on trybots to make sure things compile everywhere.

/cc @adams-sarah @cybrcodr @andybons

[bradfitz]

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.)