kbuild: add -Wundef to KBUILD_CPPFLAGS for W=1 builds

The use of an undefined macro in an #if directive is warned, but only
in *.c files. No warning from other files such as *.S, *.lds.S.

Since -Wundef is a preprocessor-related warning, it should be added to
KBUILD_CPPFLAGS instead of KBUILD_CFLAGS.

My previous attempt [1] uncovered several issues. I could not finish
fixing them all.

This commit adds -Wundef to KBUILD_CPPFLAGS for W=1 builds in order to
block new breakages. (The kbuild test robot tests with W=1)

We can fix the warnings one by one. After fixing all of them, we can
make it default in the top Makefile, and remove -Wundef from
KBUILD_CFLAGS.

[1]: https://lore.kernel.org/all/20221012180118.331005-2-masahiroy@kernel.org/

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