Emit C attributes via cbindgen

C and C++ have supported attributes in some measure for decades. Attributes offer hints to the compiler such as if a function is deprecated or doesn't return. The latest revision of the C standard, C23, consolidates some of these attributes and uses a style similar to C++.

relibc ships with a features.h header that defined macros that allowed using these attributes in a backwards compatible way. I updated the macros to account for C23's attributes.

cbindgen is able to auto emit these attributes based on Rust's attributes. For example, #[deprecated = "Use foobar instead"] turns into [[deprecated("Use foobar instead")]]. I updated relibc's global cbindgen definitions to use the macros defined in "features.h".

The only caveat is that features.h must be included where attributes such as #[must_use] are used. This is only a one line change in the respective cbindgen.toml where necessary.

I opted to rewrite the macros in features.h because relibc already had similar macros defined there from musl. However, I think it may be better to move them to a different file which is indirectly included by libc. It seems like glibc does that, but I couldn't figure out how.

Finally, some tests squelch the new warnings in order to not fail CI due to -Werror.