kbuild: fix the modules order between drivers and libs

Commit b2c88554 ("kbuild: update modules.order only when contained
modules are updated") accidentally changed the modules order.

Prior to that commit, the modules order was determined based on
vmlinux-dirs, which lists core-y/m, drivers-y/m, libs-y/m, in this order.

Now, subdir-modorder lists them in a different order: core-y/m, libs-y/m,
drivers-y/m.

Presumably, there was no practical issue because the modules in drivers
and libs are orthogonal, but there is no reason to have this distortion.

Get back to the original order.

Fixes: b2c88554 ("kbuild: update modules.order only when contained modules are updated")
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>