hurd: Fix x86_64 _hurd_tls_fork

It is illegal to call thread_get_state () on mach_thread_self (), so
this codepath cannot be used as-is to fork the calling thread's TLS.
Fortunately we can use THREAD_SELF (aka %fs:0x0) to find out the value
of our fs_base without calling into the kernel.

Fixes: f6cf701e
"hurd: Implement TLS for x86_64"

Checked on x86_64-gnu: fork () now works!

Signed-off-by: Sergey Bugaev <bugaevc@gmail.com>
Message-Id: <20230517191436.73636-8-bugaevc@gmail.com>