loader.py: show the thread name while printing all threads

It is really helpful to know who is who when you are printing a backtrace
for all threads.

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