Don't use -Wl,--fix-cortex-a8 for Cortex A7, A9, A15, Krait or Denver

-Wl,--fix-cortex-a8 is a workaround for an Erratum in Cortex-A8
processors. It slightly increases code size and decreases performance,
and there's no point in using it on non-A8 CPUs.

Instead of forcing it unconditionally, use it when targeting
Cortex-A8 or generic armv7-a (which might or might not be A8).

Change-Id: Ifa59765d380445237edccfe5440a67b3ba1e459a
Signed-off-by: Bernhard Rosenkränzer <Bernhard.Rosenkranzer@linaro.org>