remove support for extern-block const intrinsics
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 https://github.com/rust-lang/rust/issues/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`
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