kbuild: split adjust_autoksyms.sh in two parts

In order to prepare the ground for a build-time optimization, split
adjust_autoksyms.sh into two scripts: one that generates autoksyms.h
based on all currently available information (whitelist, and .mod
files), and the other to inspect the diff between two versions of
autoksyms.h and trigger appropriate rebuilds.

Acked-by: Nicolas Pitre <nico@fluxnic.net>
Tested-by: Matthias Maennich <maennich@google.com>
Reviewed-by: Matthias Maennich <maennich@google.com>
Signed-off-by: Quentin Perret <qperret@google.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>