Update cfg(stage0) references to cfg(bootstrap) instead

[scottmcm]

https://github.com/rust-lang/rustc-guide/blob/master/src/stabilization_guide.md#removing-existing-uses-of-the-feature-gate still mentions #![cfg_attr(stage0, feature(XXX))], but as of rust-lang/rust#61494, it's bootstrap instead of stage0.

[JohnTitor]

It seems this is fixed in #346.