bus/address error handler changes

Hi,

I've committed bus error and address error vector handler routines
to sync them with other m68k ports:

http://mail-index.netbsd.org/source-changes/2014/03/14/msg052742.html

As noted in the log, these changes should be harmless,
(all new handlers are used by other ports) but I tested
then only on X68030 so 040/060 handlers are not tested.

If you gets any problem on GENERIC kernel in the latest snapshot,
please report it here or via send-pr.

Thanks,

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Izumi Tsutsui