kbuild: rpm-pkg: Generate debuginfo package manually

kbuild: rpm-pkg: Generate debuginfo package manually

Commit a7c699d090a1 ("kbuild: rpm-pkg: build a debuginfo RPM") adjusted
the __spec_install_post macro to include __os_install_post, which runs
brp-strip. This ends up stripping module signatures, breaking loading
modules with lockdown enabled.

Undo most of the changes of the aforementioned debuginfo patch and
mirror commit 16c36f8864e3 ("kbuild: deb-pkg: use build ID instead of
debug link for dbg package") in kernel.spec to generate a functionally
equivalent debuginfo package while avoiding touching the modules after
they have already been signed during modules_install.

Fixes: a7c699d090a1 ("kbuild: rpm-pkg: build a debuginfo RPM")
Reported-by: Holger Kiehl <Holger.Kiehl@dwd.de>
Closes: https://lore.kernel.org/68c375f6-e07e-fec-434d-6a45a4f1390@praktifix.dwd.de/
Tested-by: Holger Kiehl <Holger.Kiehl@dwd.de>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Link: https://patch.msgid.link/20260121-fix-module-signing-binrpm-pkg-v1-1-8fc5832b6cbc@kernel.org
Signed-off-by: Nicolas Schier <nsc@kernel.org>