Implement futex support on FreeBSD (_umtx_op)

[RalfJung]

When running our test suite on FreeBSD, almost all test failures are due to missing an implementation of _umtx_op, which is the FreeBSD version of futexes. Given that we have shared futex infrastructure that backs both the Linux and Windows futex support, it shouldn't be too hard to also add support for the FreeBSD variant of this.

There was a PR at #2321 that had some issues but could be used as a starting point.

[LorrensP-2158466]

May I try to do this?

[RalfJung]

Sure. :) Apart from #2321 mentioned above, the recent PR adding futexes for macOS can probably also help serve as a starting point: #4142.

If you have any questions, our Zulip stream is a good place to ask.

Note that I am super busy currently so I do not have a lot of time for mentoring

[LorrensP-2158466]

Thanks I'll look into it.

And that's no problem, classes started so i'm also busy. If I'm really stuck I'll ask on Zulip and go from there.

@rustbot claim

[RalfJung]

This was implemented in #4209.