gdb: implement __bool__ which is required by python3

What was __nonzero__ on python2 is now __bool__ on python3. This
change fixes an issue with "osv info threads" which printed a line
starting with "timers:" even if there were no timers.

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