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>