ZeroTier has been missing for ARM targets, such as the Raspberry Pi. The issue turns out to be a pair of small configuration options involving ARM NEON (SIMD) support. I've created a pull request upstream for the correction, but just in case it becomes stalled or rejected, here are the file changes so they can potentially be added to the FreeBSD ports collection.
Vincent thanks! I'll roll the patch into https://reviews.freebsd.org/D16880 fixing a few other compiler related things, hopefully it can go in next week.