remove support for extern-block const intrinsics

[RalfJung]

This converts all const-callable intrinsics into the "new" form of a regular fn with #[rustc_intrinsic] attribute. That simplifies some of the logic since those functions can be marked const fn like regular functions, so intrinsics no longer need a special case to be considered const-callable at all.

I also added a new attribute #[rustc_const_stable_intrinsic] to mark an intrinsic as being ready to be exposed on stable. Previously we used the #[rustc_const_stable_indirect] attribute for that, but that attribute had a dual role -- when used on a regular function, it is an entirely safe marker to make this function part of recursive const stability, but on an intrinsic it is a trusted marker requiring special care. It's not great for the same attribute to be sometimes fully checked and safe, and sometimes trusted and requiring special care, so I split this into two attributes.

This also fixes #122652 by accepting intrinsics as const-stable if they have a fallback body that is recursively const-stable.

The library changes are best reviewed with whitespace hidden.

r? @compiler-errors