Querify `entry_fn` Analogous to https://github.com/rust-lang/rust/pull/57570 but this will also require few fixups in Miri so I decided to separate that (and it seems [CI doesn't let us break tools anymore](https://github.com/rust-lang/rust/pull/57392#issuecomment-453801540)? Or was that because it was a rollup PR?) r? @nikomatsakis