Rollup merge of rust-lang#139455 - Skgland:remove_rust-intrinsic_ABI, r=oli-obk

 Remove support for `extern "rust-intrinsic"` blocks

Part of rust-lang#132735

Looked manageable and there didn't appear to have been progress in the last two weeks,
so decided to give it a try.

Author: Zalathar

Diff for: core/src/intrinsics/mod.rs

@@ -8,6 +8,7 @@
 //! Note: any changes to the constness of intrinsics should be discussed with the language team.
 //! This includes changes in the stability of the constness.
 //!
+//! //FIXME(#132735) "old" style intrinsics support has been removed
 //! In order to make an intrinsic usable at compile-time, it needs to be declared in the "new"
 //! style, i.e. as a `#[rustc_intrinsic]` function, not inside an `extern` block. Then copy the
 //! implementation from <https://github.com/rust-lang/miri/blob/master/src/intrinsics> to