kbuild: make modules_install copy modules.builtin(.modinfo)

Josh Triplett reports that initramfs-tools needs modules.builtin and
modules.builtin.modinfo to create a working initramfs for a non-modular
kernel.

If this is a general tooling issue not limited to Debian, I think it
makes sense to change modules_install.

This commit changes the targets as follows when CONFIG_MODULES=n.

In-tree builds:
  make modules          -> no-op
  make modules_install  -> install modules.builtin(.modinfo)

External module builds:
  make modules          -> show error message like before
  make modules_install  -> show error message like before

Link: https://lore.kernel.org/lkml/36a4014c73a52af27d930d3ca31d362b60f4461c.1686356364.git.josh@joshtriplett.org/


Reported-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>
Tested-by: Nicolas Schier <nicolas@fjasle.eu>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Tested-by: Josh Triplett <josh@joshtriplett.org>