Design and implement what I've been calling "capability computation":

1. Let the user specify what capabilities to allow. Exactly how they do it is open right now - e.g. attributes, or just a -C target-feature temp hack.
2. Validate those capabilities are correct. Error on missing capability, warning on not-needed capability?

Note validation is surprisingly complex, e.g. some capabilities say "you can't opbitcast this particular type to that particular type", it's not just simple "this instruction is/isn't allowed" rules.

@Jasper-Bekkers is working on part 2 right now.