loader.py: make 'osv info threads' display the thread on which joiner waits

Saves a bit of manual work.

Example:

  gdb$ osv info threads
  0 (0x1de9e08) cpu0x0 waiting    sched::thread::join() at core/sched.cc (SNIP)
	joining on 0xffffc0003e41e010

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>