Blackfin: fix initial stack pointer setup

During very early init, the stack pointer is given a slightly incorrect
value (&init_thread_union).  The value is later adjusted to the right one
during early init (&init_thread_union + THREAD_SIZE), but it is used a few
times in between.  While the few functions used don't actually put things
onto the stack (due to optimization), it's best if we simply use the right
value from the start.

Signed-off-by: Barry Song <barry.song@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>