kbuild: fix false-positive need-builtin calculation

[ Upstream commit d9f78edfd81b9e484423534360350ef7253cc888 ]

The current implementation of need-builtin is false-positive,
for example, in the following Makefile:

  obj-m := foo/
  obj-y := foo/bar/

..., where foo/built-in.a is not required.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Stable-dep-of: 2e3d0e20d845 ("ARM: dts: exynos: correct TMU phandle in Odroid HC1")
Signed-off-by: Sasha Levin <sashal@kernel.org>