Add stdsimd feature to allow_internal_unstable attribute in feature detect macros

[yaahc]

This is a blocker for rust-lang/rust#95956

That PR modifies the stability checking logic of rustc so that it additionally checks the stability of the path segments used to name an item, rather than only checking the stability of the item itself. This is necessary to allow std::error::Error to be moved into a new unstable core::error module while still being available on stable via a re-export in the stable std::error module.

That change breaks the feature detection macros since they start encountering the stdsimd unstable feature which is not explicitly mentioned in those macros allow_internal_unstable attribute.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @Amanieu (or someone else) soon.

Please see the contribution instructions for more information.

[yaahc]

no idea what's up with that test failure...

[ChrisDenton]

This is probably caused by rust-lang/rust#95604, which lead to some issues with stdcall symbol name mangling. The potential fix is rust-lang/rust#96444