* `futex2_waitv` always takes a 64-bit timespec. Perhaps the
`kernel_timespec` should be renamed `timespec64`? Its used in iouring,
too.
* Add `packed struct` for futex v2 flags and parameters.
* Add very basic "tests" for the futex v2 syscalls (just to ensure the
code compiles).
* Update the stale or broken comments. (I could also just delete these
they're not really documenting Zig-specific behavior.)
Given that the futex2 APIs are not used by Zig's library (they're a bit
too new), and the fact that these are very specialized syscalls, and they
currently provide no benefit over the existing v1 API, I wonder if instead
of fixing these up, we should just replace them with a stub that says 'use
a 3rd party library'.