I think few would define or understand 'freedom' as 'the right to do whatever you want'. What if what you want is physically impossible, such as travelling faster than the speed of light or jumping unaided over the Empire State building ?
But let's get closer to a philosophical analysis.
Freedom as non-constraint or absence of impediment
For one thing, freedom is a state of the individual person or group in relation to certain constraints or impediments. A free person has definitionally certain options and capacities for action. I am free if I can get out of the room in which I am locked, say, because there are no intervening constraints or impediments that actually prevent me. This has nothing to do directly with my right - my moral entitlement - to get out of the room. Perhaps I have no such right or entitlement because e.g. I am serving a just punishment or have promised to remain in the room and not to try to exit.
Freedom and human agency
This is a wide construction of 'freedom'. Usually when we talk of freedom the relevant constraints or impediments are those imposed or removable by human agency. I am not free to cross the road if you physically and intentionally block my way or put me under coercive threat or in the original situation if you know that I am unwillingly locked in a room and will not let me out. You reduce my freedom by what you do (on the road) or don't do (unlock the door).
Freedom and rights
We are as yet nowhere near the idea of freedom as a moral right. (I set aside legal rights since moral rights seem most relevant to your question.) It is only in the context of respect for autonomy - for a person's human dignity as an agent - or of a doctrine of human rights that freedom acquires any linkage with the notion of entitlement to act.
In such a moralised context freedom as a right is limited by the rights of others. Not only do I have human rights - you do, too. If I have the right to cross the road without constraint or impediment, so do you: and therefore freedom, at the point the argument has reached, can never be pared down to 'the right to do whatever you want' in the sense of what you merely plan or prefer intentionally to do. The parameters of my rights to freedom of action are your rights, and vice versa. There is no suggestion here of a psychopathic freedom in which nobody else but the agent counts.
Freedom, rights, and harm
The danger is, of course, that you can nullify my freedom because for every right I have, you may have a counter-right or a right that may collide with mine. I have the right to cross the road at point X and time t1. But so have you.
I favour a position such as John Stuart Mill's in which a harm principle applies to freedom (Mill, On Liberty, 1859, Introduction). I am free to act - I have that moral entitlement - only if in exercising my right I cause you no harm or cannot reasonably foresee harm. (The concept of harm needs tightening but the basic idea is intuitively clear.)
The subjectivity of morals
You still have an opening for reply. For you hold that: 'the notion of moral principles and responsibilities is well known to be subjective.'
The subjectivity of moral principles and responsibilities is, whatever else it may be, not known to be subjective. The question whether morality is subjective (for the record, in what sense precisely?) or objective so that some moral principles are known to be true as moral realists hold or, with Kantian objectivists, such principles are deliverances of reason, is an open question. I'm not going to try to close it here but you cannot lay down as axiomatic or an evident truth that (to repeat your phrase) 'the notion of moral principles and responsibilities is well known to be subjective.' Any such conclusion can be reached, if at all, only at the end of far more philosophical argument than you offer. In fact, you don't argue but only state. I don't want to sound too heavy but that lacuna in your question has to be pointed out.
Interesting question, though; if it hadn't been, I wouldn't have answered it. I look forward to your next appearance on PSE.