Support swapping per-thread fs register value

Briefly noting an idea we discussed IRL: since our main usage for storing fs register value is to support a swap, we can maintain a (global, rwlocked) hashmap that tells us the swaps. We would need to set up a state-machine-like thing to leave values "unlinked" until we find the link (during libc initialization), but generally speaking, the idea seems feasible.

@CvvT is also exploring some pthread internals to see if they might be helpful as an alternative, just in case that ends up simpler, but yeah, we have ways to solve this :)

Originally posted by @jaybosamiya-ms in https://github.com/MSRSSP/litebox/issues/56#issuecomment-2759672534