make pp_return() use leave_adjust_stacks()

make pp_return() use leave_adjust_stacks()

It was using S_leave_common(), but that's shortly to be removed.  It also
required adding an extra arg to leave_adjust_stacks() to indicate where to
shift the return args to. This will also be needed for when we replace the
remaining uses of  S_leave_common() with leave_adjust_stacks().