add jitted attr for stable arches of repo objects

Basically just interates through a repo's defined profiles in the
profiles.desc file and returns the set of arches that have at least one
stable profile. Note that it skips all hypenated arches for now. Only
amd64-fbsd has a single stable profile for some reason, but we shouldn't
care about it by default.

This will help us define better default arch lists for stable arch only
checks in pcheck.