Arms aren't the only area where the Reagan administration is keen on ''Bargaining chips.'' The Agriculture Department, it seems, wants a few of its own as the US resumes trade talks with the Europeans next week. To prod Europe into reducing its fram-export subsidies, the department proposes such moves as quickly dumping some American butter on the world market or subsidizing specified US farm exports. Seems a strange way to warm up the US-European atmosphere. We hompe more sober heads prevail.