Blackfin: add support for the DBGA (debug assert) pseudo insn

A few pseudo debug insns exist to make testing of simulators easier.
Since these don't actually exist in the hardware, we have to have the
exception handler take care of emulating these.  This allows sim test
cases to be executed unmodified under Linux and thus simplify debugging
greatly.

Signed-off-by: Robin Getz <robin.getz@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>