hurd: Fix using interposable hurd_thread_self

Create a private hidden __hurd_thread_self alias, and use that one.

Fixes 2f8ecb58
"hurd: Fix x86_64 _hurd_tls_fork" and
c7fcce38
"hurd: Make sure to not use tcb->self"

Reported-by: Joseph Myers <joseph@codesourcery.com>
Signed-off-by: Sergey Bugaev <bugaevc@gmail.com>