arm: fix build with gcc 4.7.x

arm: fix build with gcc 4.7.x

As was already pointed out months ago (see
http://lists.xen.org/archives/html/xen-devel/2012-02/msg00826.html),
gcc 4.7.x (imo validly) refuses to take both -mcpu=cortex-a15 and
-march=armv7-a due to conflicting feature sets causing amibiguity in
instruction selection. Since the former implies the latter, just use
the former (and drop the -march=).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>