kbuild: Generate modules.builtin in make modules

Generating the file in make modules_install was broken as well, because
it didn't work in a readonly filesystem and otherwise it generated a
root-owned file which is not wanted.

Reported-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Michal Marek <mmarek@suse.cz>