timerfd on FreeBSD >= 14.0

Same API as Linux. See [timerfd(2)](https://man.freebsd.org/cgi/man.cgi?query=timerfd&sektion=2&manpath=FreeBSD+15.0-CURRENT) and [`<sys/timerfd.h>`](https://github.com/freebsd/freebsd-src/blob/main/sys/sys/timerfd.h) for details.

Similar to `eventfd` the ABI is new, so no need to gate by `cfg(freebsd14)`.
