Assange doesn't support Trump or the Republicans. He supports Hillary Clinton not becoming President, because of his personal vendetta against her. It just happens that his efforts to undermine the Clinton campaign help Trump.

As an anarchist, Assange probably wouldn't care about the libertarians more than any other candidate, since any candidate would only futher the existence and legitimacy of a state which he would rather see destroyed. If he did support Trump, it would be because he believes a Trump Presidency would only hasten America's demise.