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

commit 6748cb3c299de1ffbe56733647b01dbcc398c419 upstream.

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>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>