Rollup merge of #128596 - RalfJung:const_fn_floating_point_arithmetic, r=nnethercote

stabilize const_fn_floating_point_arithmetic

Part of rust-lang/rust#128288
Fixes rust-lang/rust#57241

The existing test `tests/ui/consts/const_let_eq_float.rs`  ([link](https://github.com/RalfJung/rust/blob/const_fn_floating_point_arithmetic/tests/ui/consts/const_let_eq_float.rs)) covers the basics, and also Miri has extensive tests covering the interpreter's float machinery. Also, that machinery can already be used on stable inside `const`/`static` initializers, just not inside `const fn`.

This was explicitly called out in rust-lang/rfcs#3514 so in a sense t-lang just recently already FCP'd this, but let's hear from them whether they want another FCP for the stabilization here or whether that was covered by the FCP for the RFC.
Cc ``@rust-lang/lang``

### Open items

- [x] Update the Reference: rust-lang/reference#1566

Author: matthiaskrgr