miri: Add option to flag uninitialized integers as UB

The reference currently states that this code is UB, but Miri deliberately accepts it:

fn main() {
    let _val = unsafe { std::mem::MaybeUninit::<usize>::uninit().assume_init() };
}

The reason we accept it is that the lang-team is not sure if we really want this to be UB (also see https://github.com/rust-lang/unsafe-code-guidelines/issues/71), so the conservative choice for the reference is to make it UB for now, but in Miri I fear this might lead to too many errors that people could consider false positives (even though by the letter of the reference they are true positives).

Still it could be interesting to see how much code (that Miri can run) actually fails when considering uninitialized integers UB, so having a flag in Miri to enable stricter checking would be interesting.