I’m surprised it’s not in the OED. The earliest citation I can find in a cursory search is in Walter Scott’s 1810 poem “The Lady of the Lake.” A knight proclaims that he will not give way before his foes:
“Come one, come all! this rock shall fly
From its firm base as soon as I.”
I’m fairly certain that antedates can be found.
Since English only has a defective subjunctive mood, calling this subjunctive would be problematic—grammatically the subjunctive, imperative, and indicative would not differ in this case. Depending on the context, one could call it hortatory subjunctive if the speaker includes himself, and a straightforward imperative if not. In the Scott poem, it would be imperative, as the speaker is not going to fight himself.