Blackfin: work around anomaly 05000220

When possible, work around anomaly 05000220 (external memory is write
back cached, but L2 is not cached).  If not possible, detect the
conditions at build time and reject any qualifying configurations.

Signed-off-by: Graf Yang <graf.yang@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>