Rollup merge of #101131 - RalfJung:ctfe-no-needs-rfc, r=oli-obk

CTFE: exposing pointers and calling extern fn is just impossible

The remaining "needs RFC" errors are just needlessly confusing, I think -- time to get rid of that error variant. They are anyway only reachable with miri-unleashed (if at all).

r? `@oli-obk`

compiler/rustc_const_eval/src/interpret/machine.rs

@@ -490,6 +490,7 @@ pub macro compile_time_machine(<$mir: lifetime, $tcx: lifetime>) {
     ) -> InterpResult<$tcx, Pointer<Option<AllocId>>> {
         // Allow these casts, but make the pointer not dereferenceable.
         // (I.e., they behave like transmutation.)
+        // This is correct because no pointers can ever be exposed in compile-time evaluation.
         Ok(Pointer::from_addr(addr))
     }