Enable cfg checking of cargo features for everything but std

[Urgau]

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

[Mark-Simulacrum]

@bors r+ rollup=iffy

[bors]

📌 Commit 81d2cba has been approved by Mark-Simulacrum

[bors]

⌛ Testing commit 81d2cba with merge ea92b08...

[bors]

☀️ Test successful - checks-actions
Approved by: Mark-Simulacrum
Pushing ea92b08 to master...

[rust-timer]

Finished benchmarking commit (ea92b08): comparison url.

Summary: This benchmark run did not return any relevant results.

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

@rustbot label: -perf-regression