Make `rustc_on_unimplemented` std-agnostic for `alloc::rc`
See https://github.com/rust-lang/rust/issues/112923
Just a few lines related to `alloc:rc` for `Send` and `Sync`.
That seems to be all of the `... = "std::..."` issues found, but there a few notes with `std::` inside them still.
r? `@WaffleLapkin`