Expose Linux specifics and libnuma interop

[HadrienG2]

Expose Linux-specific API functions and libnuma interoperability behind suitable cfg flags with associated doc(cfg)/cfg(doc):

• [x] Linux: https://hwloc.readthedocs.io/en/v2.9/group__hwlocality__linux.html
• [ ] libnuma:
  • ulong: https://hwloc.readthedocs.io/en/v2.9/group__hwlocality__linux__libnuma__ulongs.html
  • bitmask: https://hwloc.readthedocs.io/en/v2.9/group__hwlocality__linux__libnuma__bitmask.html

libnuma support should also be gated behind a suitable feature, and will depend on the availability of a minimally working binding.

[HadrienG2]

The Linux part of this ~~should be done soon (hopefully next week)~~ is done, but the libnuma part requires libnuma bindings whose masks expose *const c_ulong interop. The current Rust libnuma bindings don't do this and seem unmaintained, so before I tackle this issue, someone else must take over the maintenance of Rust libnuma bindings and expose this functionality.