Rollup merge of #77122 - ecstatic-morse:const-fn-arithmetic, r=RalfJung,oli-obk

Add `#![feature(const_fn_floating_point_arithmetic)]`

cc #76618

This is a template for splitting up `const_fn` into granular feature gates. I think this will make it easier, both for us and for users, to track stabilization of each individual feature. We don't *have* to do this, however. We could also keep stabilizing things out from under `const_fn`.

cc @rust-lang/wg-const-eval
r? @oli-obk

compiler/rustc_span/src/symbol.rs

@@ -352,6 +352,7 @@ symbols! {
         const_evaluatable_checked,
         const_extern_fn,
         const_fn,
+        const_fn_floating_point_arithmetic,
         const_fn_transmute,
         const_fn_union,
         const_generics,