Stallman's politics cannot be easily captured with a label. Here he is endorsing the most leftist well-known candidates for US president and British prime minister: To put an end to this oppression of workers, vote for Sanders and Corbyn.
He is the opposite of a libertarian. Copyleft requires, supports, validates, the state.
An actual libertarian would be pro: "there's no copyright nor copyleft; my computer -> my rules, I do whatever I want with whatever files I empirically have. And the only way to decisively stop me from doing that is to actually kill me."
Libertarians don't oppose the state, they just want its function constrained far, far below what any modern government offers; contract law and intellectual property often fall under roles libertarians consider suitable for government.
I'd say he's more communist, since he seems to believe that private property just shouldn't exist. Libertarians by contrast would tend more on the scale of "you should be able to do whatever you want with your private property."
> since he seems to believe that private property just shouldn't exist
Although I'm sympathetic to that idea myself, that just isn't true in Stallman's case. He's a Bernie/Stein voter and he's read something by Emma Goldman, but I think that's the extent of his "lefty" gland.