On FreeBSD, set the ELF symbol version for readdir_r

This function will probably be removed in FreeBSD 16.  But the old
symbol will remain, for backwards-compatibility.  Set the symbol version
now, so that the current version of libc will still be able to compile
on future versions of FreeBSD.

(backport <#4694>)
(cherry picked from commit 3908285)

Author: asomers

src/unix/mod.rs

@@ -1745,6 +1745,10 @@ cfg_if! {
                 all(target_os = "freebsd", any(freebsd11, freebsd10)),
                 link_name = "readdir_r@FBSD_1.0"
             )]
+            #[cfg_attr(
+                all(target_os = "freebsd", not(any(freebsd11, freebsd10))),
+                link_name = "readdir_r@FBSD_1.5"
+            )]
             #[allow(non_autolinks)] // FIXME(docs): `<>` breaks line length limit.
             /// The 64-bit libc on Solaris and illumos only has readdir_r. If a
             /// 32-bit Solaris or illumos target is ever created, it should use