kbuild: read *.mod to get objects passed to $(LD) or $(AR)

ld and ar support @file, which command-line options are read from.

Now that *.mod lists the member objects in the correct order, without
duplication, it is ready to be passed to ld and ar.

By using the @file syntax, people will not be worried about the pitfall
described in the NOTE.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>