teach miri to use intrinsics' fallback bodies

[oli-obk]

And then replace lots of our SIMD and numeric shims with fallback bodies by adding said fallback bodies to libcore.

Note that we need to take lots of care to preserve UB.

This may regress diagnostics quality, so maybe we need some sort of unreachable_unchecked intrinsic equivalent which can be passed a string that explains the UB?

[RalfJung]

I think we may want to require some sort of opt-in here, like #[rustc_intrinsic_fallback_checks_ub] or so, so that one can locally review in the intrinsic body whether the requirement is upheld.

[RalfJung]

Isn't this a duplicate of #3302?