Blackfin arch: Fix random crash issue found by Michael.

This is fixes a problem where we could jump to the wrong address. By
doing a "p0 = reti; jump (p0)". If a different, higher level interrupt
came in, just before, rather than returning to the calling function, we
would return to a random place in the kernel.

This very elegant fix from Bernd grabs the return location off the
stack, and places it into P0, so when we do a return, it goes to the
correct place.

Signed-off-by: Robin Getz <robin.getz@analog.com>
Signed-off-by: Bryan Wu <bryan.wu@analog.com>