Add `recvmmsg` and `sendmmsg` on OpenBSD (*-unknown-openbsd)

`recvmmsg` is supported on OpenBSD since OpenBSD 7.2: https://www.openbsd.org/72.html
Same for `sendmmsg`.

Documentation is here:
- https://man.openbsd.org/OpenBSD-7.2/recvmmsg
- https://man.openbsd.org/OpenBSD-7.2/sendmmsg

Related PR made at the time when OpenBSD did not support these functions yet: https://github.com/rust-lang/libc/pull/1231