FreeBSD: clean up readdir_r

Once https://github.com/rust-lang/libc/pull/3723 propagates into the libc used by std, we should be able to clean up [this](https://github.com/rust-lang/miri/blob/b2ac3969059891e63bb98bbc6f7f3209e96e3047/src/shims/unix/fs.rs#L1034-L1043) as the same freebsd APIs will be available when std is built on FreeBSD vs when it is cross-built by Miri.