scripts: teach extract-vmlinux about LZ4 and ZSTD

Note that the LZ4 signature is different than that of modern LZ4 as we
use the "legacy" format which suffers from some downsides like inability
to disable compression.

Signed-off-by: Adam Borowski <kilobyte@angband.pl>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>