Blackfin arch: fix bug - kernel with SMP patch can not bootup

The original code defined _exception_stack but not alloc space for the exception
stack. In exception, this area is over written by exception stack. Common kernel
luckly boot up, but SMP kernel stuck.

Signed-off-by: Graf Yang <graf.yang@analog.com>
Signed-off-by: Bryan Wu <cooloney@kernel.org>