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>

arch/blackfin/mach-common/entry.S

@@ -1555,7 +1555,8 @@ _last_cplb_fault_retx:
 .bss
 #endif
 _exception_stack:
-.size _exception_stack, 1024 * 4
-.set _exception_stack_top, _exception_stack + 1024 * 4
-.size _exception_stack_top, 0
+	.rept 1024
+	.long 0
+	.endr
+_exception_stack_top:
 #endif