Allow -mcpu=cortex-a15 for clang

Clang 3.6 (including the snapshot present in AOSP) added support for
-mcpu=cortex-a15 -- there's no need to fall back to generic armv7-a
anymore.

Signed-off-by: Bernhard Rosenkränzer <Bernhard.Rosenkranzer@linaro.org>
Signed-off-by: Maxime Poulain <maxime.poulain@student.uclouvain.be>

Change-Id: I8e1ddc04a6046355c732c35d690453f61ecb206a