x/build: add to trybot set based on port-specific files

[rsc]

In #53383, @laboger suggests that, for example, if a CL touches a file that only builds in ppc64le, we could add a ppc64le builder to the trybot set automatically. We should investigate whether this is practical.

[dmitshur]

Issue #42661 is related, it's about automatically opting-in to longtest builders when a change is deemed to modify the cmd/go command or its tests.

CC @golang/release.

[prattmic]

I can't find the issue, but we've also discussed running only affected tests. e.g., if I change debug/elf it isn't necessary to run encoding/json tests.

This is a similar vein taken to the logical extreme by applying to GOOS/GOARCH, plus adding tests that are otherwise disabled by default. I like the idea.