BACKPORT: FROMLIST: 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.

[ BACKPORT: 60ae1b194b45 not present, sed 3rd line of modules.order ]

Bug: 148277666
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>
Link: https://lore.kernel.org/lkml/20200218094139.78835-3-qperret@google.com
Change-Id: I7ed4f8f72c0ef9ac98419877efb5ef828d10b0d7