sh: Correct the offset of the return address in ret_from_exception

The address that ret_from_exception and ret_from_irq will return to is
found in the stack slot for SPC, not PR. This error was causing the
DWARF unwinder to pick up the wrong return address on the stack and then
unwind using the unwind tables for the wrong function.

While I'm here I might as well add CFI annotations for the other
registers since they could be useful when unwinding.

Signed-off-by: Matt Fleming <matt@console-pimps.org>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>