scripts/gdb: add lx_thread_info_by_pid helper

scripts/gdb: add lx_thread_info_by_pid helper

The tasks module already provides helpers to find the task struct by
pid, and the thread_info by task struct; however this is cumbersome to
utilise on the gdb commandline.

Wrap these two functionalities together in an extra single helper to
allow exploring the thread info, from a PID value

Link: http://lkml.kernel.org/r/dadc5667f053ec811eb3e3033d99d937fedbc93b.1462865983.git.jan.kiszka@siemens.com
Signed-off-by: Kieran Bingham <kieran.bingham@linaro.org>
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>