BACKPORT: FROMLIST: kbuild: generate autoksyms.h early

When doing a cold build, autoksyms.h starts empty, and is updated late
in the build process to have visibility over the symbols used by in-tree
drivers. But since the symbol whitelist is known upfront, it can be used
to pre-populate autoksyms.h and maximize the amount of code that can be
compiled to its final state in a single pass, hence reducing build time.

Do this by using gen_autoksyms.sh to initialize autoksyms.h instead of
creating an empty file.

[ 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/20200218094139.78835-4-qperret@google.com
Change-Id: I98f3394b190f097390f980e5d4d57981f91b22a6