Blackfin: unify memory region checks between kgdb and traps

The kgdb (in multiple places) and traps code developed pretty much
identical checks for how to access different regions of the Blackfin
memory map, but each wasn't 100%, so unify them to avoid duplication,
bitrot, and bugs with edge cases.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>