Blackfin arch: fix bug - traps test case 19 for exception 0x2d fails

Enable null pointer checking for ICPLBs. The code was there but for
some reason I had commented it out at some stage during development.

Should restrict this to 1K since atomic ops start there.

Signed-off-by: Bernd Schmidt <bernds_cb1@t-online.de>
Signed-off-by: Bryan Wu <cooloney@kernel.org>