UPSTREAM: kbuild: use -Oz instead of -Os when using clang

This generates smaller resulting object code when compiled with clang.

Signed-off-by: Behan Webster <behanw@converseincode.com>
Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
(cherry picked from commit 6748cb3c)
Signed-off-by: Greg Hackmann <ghackmann@google.com>

Change-Id: I5336ef3af6c7c638d9a68661c3c0e3f22693fdc8