scripts: add extract-vmlinux

This script can be used to extract vmlinux from a compressed
kernel image (bzImage, etc..). It's inspired from (a subset of)
extract-ikconfig.

It's something a lot of people have been looking for (mainly
people with xen < 4 that doesn't support bzImages at all).

Signed-off-by: Corentin Chary <corentincj@iksaif.net>
Signed-off-by: Michal Marek <mmarek@suse.cz>