Put basic impls for f16 and f128 behind cfg(not(bootstrap)) We will lose `f16` and `f128` in the beta compiler after the revert for <https://github.com/rust-lang/rust/issues/123282> lands. Change what was added in <https://github.com/rust-lang/rust/pull/123085> to be behind `#[cfg(not(bootstrap))]` to account for this.