Don't use SPR_NPC for determining exception vector #8

We should not use the SPR_NPC here: https://github.com/openrisc/newlib/blob/or1k/libgloss/or1k/crt0.S#L153

Software should instead always use l.jal and r9, but here it is even simpler: Use the id from the macro. Also needs change in the exception handler then (no address given anymore).