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.