Add attribute macro for pageable functions

Typically when you mark a function as pageable with `#[link_section = "PAGE"]`, you also call the `paged_code!` macro in the function to assert that `IRQL <= APC` in debug builds. We should just provide an attribute macro for this convenience. It also helps automatically do this good practice for IRQL checking pageable functions. This probable belongs in the `wdk` crate.