Rollup merge of rust-lang#105434 - nbdd0121:lib, r=thomcc

Fix warning when libcore is compiled with no_fp_fmt_parse

Discovered when trying to compile Rust-for-Linux with Rust 1.66 beta.

It'll be helpful if this is backported to beta (should be trivial enough for backporting), so Rust-for-Linux's rust version bump wouldn't need to do `--cap-lints allow` for libcore.