Ultra64: three sources share GioExp0 with no OR; one ack can drop another's interrupt #263

Ultra64 drives IocInterrupt::GioExp0 from three sources (gio_int, rdb_r, rdb_w; ultra64.rs ~195/207/223) and clears it on any one source's ack (~486/520/532). There is no OR between them, so acknowledging one source drops the line while another may still be pending, and that interrupt is lost until it fires again.

A fix would track the three pending flags and drive the line with their OR. Found while moving Ultra64 onto an IrqLine (#261), which keeps today's behaviour.

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