scripts/gdb: add ps command

Signed-off-by: Thiébaud Weksteen <thiebaud@weksteen.fr>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>