Support atexit, and detect racy calls to `exit` as UB

[RalfJung]

See rust-lang/rust#126600, rust-lang/rust#126606. However I am not sure how to even best do that... we just atomically stop the machine on exit currently. Instead we'd have to keep other threads going for a while and see whether any of them also calls exit, and then report UB?

We also currently don't support atexit handlers, so probably the UB-inducing programs can't even be executed. So I think we're not missing UB, currently.