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.

Change-Id: I8a582261ac35e32f8f1def1d244cf83f2096520d
Signed-off-by: Bernhard Rosenkränzer <Bernhard.Rosenkranzer@linaro.org>