support for FreeBSD

Hi folks! In trying to compile a project that depends on this library on FreeBSD, I hit a stumbling block. Namely, that this library doesn't support FreeBSD.

I'm seeing a lot of commits that say "(re)generate", but nothing in HACKING explains how to add a new OS/platform combination.

At Tier 1, FreeBSD supports i386 (11 and 12), amd64, and aarch64 (13+)