- Add syscall bindings/structures for the `futex2` family.
  The documentation is taken from the syscall definitions.
- Add documnentation for the `cachestat` bindings and structures.
  Taken from work I did in Cosmopolitian libc.
- Add binding for `map_shadow_stack`.
  No documentation for this one, since the kernel devs didn't bother to
  do it ¯\_(ツ)_/¯.