support for FreeBSD
Hi folks!
In trying to compile a [project that depends on this library](https://github.com/superseriousbusiness/gotosocial/issues/184) on FreeBSD,
I hit a stumbling block. Namely, that this library doesn't support FreeBSD.
I'm seeing a lot of [commits](https://gitlab.com/cznic/libc/-/commits/master) that say "(re)generate", but nothing in [HACKING](https://gitlab.com/cznic/libc/-/blob/master/HACKING) explains how to add a new OS/platform combination.
At Tier 1, [FreeBSD supports](https://www.freebsd.org/platforms/) i386 (11 and 12), amd64, and aarch64 (13+)
issue