Fixes arcade machine runtime from #6351

This fixes runtime 7 from #6351. It was actually worse than "just a runtime" as it made the two arcade machines that spawn on the station come with no actual circuit board (just the type) so if you disconnected the display you couldn't get it back together, and if you disassembled the machine you couldn't get the circuit board out at all.

I made an issue at #6663 before realizing it was the same one as 7 in #6351, although mine has more information if you're curious.
Suggested New() function :
```
 /obj/machinery/computer/arcade/New()
	..()
	
	if(!circuit)
		var/choice = pick(typesof(/obj/item/weapon/circuitboard/arcade) - /obj/item/weapon/circuitboard/arcade)
		var/obj/item/weapon/circuitboard/CB = new choice()
		new CB.build_path(loc, CB)
		qdel(src)
```