Extend ptr::null and null_mut to all thin (including extern) types

Fixes https://github.com/rust-lang/rust/issues/93959

This change was accepted in https://rust-lang.github.io/rfcs/2580-ptr-meta.html

Note that this changes the signature of **stable** functions.
The change should be backward-compatible, but it is **insta-stable**
since it cannot (easily, at all?) be made available only
through a `#![feature(…)]` opt-in.

The RFC also proposed the same change for `NonNull::dangling`,
which makes sense it terms of its signature but not in terms of its implementation.
`dangling` uses `align_of()` as an address. But what `align_of()` should be for
extern types or whether it should be allowed at all remains an open question.

This commit depends on https://github.com/rust-lang/rust/pull/93977, which is not yet
part of the bootstrap compiler. So `#[cfg]` is used to only apply the change in
stage 1+. As far a I know bounds cannot be made conditional with `#[cfg]`, so the
entire functions are duplicated. This is unfortunate but temporary.

Since this duplication makes it less obvious in the diff,
the new definitions differ in:

* More permissive bounds (`Thin` instead of implied `Sized`)
* Different implementation
* Having `rustc_allow_const_fn_unstable(ptr_metadata)`

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