DEV: dev/gdb: add simple thread dump

DEV: dev/gdb: add simple thread dump

The thread_dump function dumps the list of known threads and a few info
on them (pointer, current run queue, flags etc). This should help more
easily spot a particular one and find stuck ones.

E.g:

  (gdb) thread_dump
  Tid    0: pth=0x7ffff7e797c0 mono=2222322327950732 now_ms=4294947291 fl=0x38 rq=-1 cq=0 current=(nil)
  Tid    1: pth=0x7ffff78d8640 mono=2222322327928085 now_ms=4294947291 fl=0x38 rq=-1 cq=0 current=(nil)
  Tid    2: pth=0x7ffff6b7e640 mono=2222322327927150 now_ms=4294947291 fl=0x38 rq=-1 cq=0 current=(nil)
  Tid    3: pth=0x7ffff637d640 mono=2222322327924878 now_ms=4294947291 fl=0x38 rq=-1 cq=0 current=(nil)
  Tid    4: pth=0x7ffff5b7c640 mono=2222322327925676 now_ms=4294947291 fl=0x38 rq=-1 cq=0 current=(nil)
  Tid    5: pth=0x7ffff537b640 mono=2222322327929524 now_ms=4294947291 fl=0x38 rq=-1 cq=0 current=(nil)
  Tid    6: pth=0x7ffff4b7a640 mono=2222322327926817 now_ms=4294947291 fl=0x38 rq=-1 cq=0 current=(nil)
  Tid    7: pth=0x7fffdffff640 mono=2222322327947960 now_ms=4294947291 fl=0x38 rq=-1 cq=0 current=(nil)