Support for futex

[michael-schwarz]

I stumbled across a paper describing support for futexes in the SPIN model checker (https://www.doc.ic.ac.uk/~afd/homepages/papers/pdfs/2023/SPIN.pdf).

We maybe also should take another look at what futexes do, and whether we can come up with interesting properties we want to check using Abstract Interpretation. This could, e.g., be a Bachelor's or Master's thesis.