gdb: make "osv info threads" work again

After commit dc40b49eb3342748b76f38a385eed6aee5b19d25, the place of
the _cpu and _status fields of a thread has moved (into a structure
pointed by the _detached_thread field). So fix loader.py to use
these new locations, so "osv info threads" will work again.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>