Blackfin: fix SMP build error in start_thread()

Commit d5ce528c (Blackfin: convert irq/process to asm-generic)
incorrectly merged the smp and non-smp cases of start_thread() causing the
L1 stack to be setup on the SMP port instead of the UP port.

Signed-off-by: Graf Yang <graf.yang@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>