Neutral shim mirroring the used subset of the x86_64 crate
(address types, page sizes, paging flags). Available on all arches:
on x86_64 it re-exports the real crate types, elsewhere it provides
neutral equivalents.
Architecture-neutral shim mirroring the subset of the x86_64 crate API
used by shared kernel code (address types, page sizes, paging flags).