Auto merge of #96670 - Urgau:bootstrap-check-cfg-features, r=Mark-Simulacrum

Enable cfg checking of cargo features for everything but std

This PR enable `cfg` checking of cargo features for everything but std, it also adds a `FIXME` to myself.

> Note: `std`, `alloc` and `core` imports some dependencies by #[path] (like
> backtrace, core_simd, std_float, ...), those dependencies have their own features
> but cargo isn't involved in the #[path] and so cannot pass the complete list of
> features, so for that reason we don't enable checking of features for std.

r? `@Mark-Simulacrum`

diff --cc src/bootstrap/builder.rs
Simple merge