kbuild: make multi_depend work with targets in subdirectory

Precisely speaking, when you get the stem of the path, you should use
$(patsubst $(obj)/%,%,...) instead of $(notdir ...).

I do not see this usecase, but if you create a composite object in a
subdirectory, the Makefile should look like this:

obj-$(CONFIG_FOO) += dir/foo.o
dir/foo-objs := dir/foo1.o dir/foo2.o

The member objects should be assigned to dir/foo-objs instead of
foo-objs.

This syntax is more consistent with commit 54b8ae66ae1a ("kbuild:
change *FLAGS_<basetarget>.o to take the path relative to $(obj)").

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