Add CI test that `#[cfg]` tags are from a defined set

Add CI test that `#[cfg]` tags are from a defined set

Rust is fairly relaxed in checking the validity of arguments
passed to #[cfg]. While it should probably be more strict when
checking features, it cannot be strict when checking loose cfg
tags, because those can be anything and are simply passed to rustc
via unconstrained arguments.

Thus, we do it for rustc manually, but scanning all our source and
checking that all our cfg tags match a known cfg tag.

Fixes #2184