hurd: i386 TLS tweaks

* Micro-optimize TLS access using GCC's native support for gs-based
  addressing when available;
* Just use THREAD_GETMEM and THREAD_SETMEM instead of more inline
  assembly;
* Sync tcbhead_t layout with NPTL, in particular update/fix __private_ss
  offset;
* Statically assert that the two offsets that are a part of ABI are what
  we expect them to be.

Signed-off-by: Sergey Bugaev <bugaevc@gmail.com>
Message-Id: <20230214173722.428140-2-bugaevc@gmail.com>