ARM: 8767/1: add support for building ARM kernel with clang

Use cc-options call for compiler options which are not available
in clang. With this patch an ARMv7 multi platform kernel can be
successfully build using clang (tested with version 5.0.1).

Based-on-patches-by: Behan Webster <behanw@converseincode.com>

Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>