convert intrinsic shims to intrinsics with fallback bodies

[oli-obk]

Now that rustc supports rust-lang/rust#93145 we can add support for it to miri and start removing some shims, writing them in pure rust in libcore instead

[RalfJung]

Note that this is only okay if the fallback body has exactly as much UB as the specification of the intrinsic permits.

We need to make sure that we don't accidentally run a fallback body that fails to detect some UB.

[oli-obk]

blocked on #3397