Blackfin arch: fix bug which unaligns the init thread's stack and causes the current macro to fail.

switch to using proper defines this time (THREAD_SIZE and PAGE_SIZE)
instead of just PAGE_SIZE everywhere

Signed-off-by: Mike Frysinger <michael.frysinger@analog.com>
Signed-off-by: Bernd Schmidt <bernd.schmidt@analog.com>
Signed-off-by: Bryan Wu <bryan.wu@analog.com>