kbuild: generate a list of objects in vmlinux

A *.mod file lists the member objects of a module, but vmlinux does
not have such a file.

Generate this list to allow modpost to know all the member objects.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>
Tested-by: Nathan Chancellor <nathan@kernel.org>