FROMGIT: kheaders: explicitly define file modes for archived headers

Build environments might be running with different umask settings
resulting in indeterministic file modes for the files contained in
kheaders.tar.xz. The file itself is served with 444, i.e. world
readable. Archive the files explicitly with 744,a+X to improve
reproducibility across build environments.

--mode=0444 is not suitable as directories need to be executable. Also,
444 makes it hard to delete all the readonly files after extraction.

Cc: <stable@vger.kernel.org>
Cc: <linux-kbuild@vger.kernel.org>
Cc: Masahiro Yamada <masahiroy@kernel.org>
Cc: Joel Fernandes <joel@joelfernandes.org>
Link: https://lore.kernel.org/r/20240528113243.827490-2-maennich@google.com
(cherry picked from commit 3bd27a84 https://git.kernel.org/pub/scm/linux/kernel/git/masahiroy/linux-kbuild.git

 kbuild)
Bug: 342094847
Bug: 342393806
Change-Id: Ib206a6e0abfacf8132bfad8c43a62982062175fa
Signed-off-by: Matthias Maennich <maennich@google.com>