Clean up items accidentally stabilized through unstable paths

[tmandry]

rustc_allowed_through_unstable_modules special cases accidental stabilizations of stable items through unstable paths.

These might be prime candidates for cleaning up over an edition, assuming they are able to be accessed through some other path. This issue exists to track that work.

core::intrinsics (attribute added in #95956, #99288)

• core::intrinsics::copy_nonoverlapping
• core::intrinsics::copy
• core::intrinsics::write_bytes
• core::intrinsics::transmute