Hi!
First, regarding a solution to target FreeBSD 12.x (with the ABI breaking change around stat(2)), there is an ongoing discussion about a future RFC to extend target triples.
About the rust-nightly port on FreeBSD, I didn’t have time yet to update it. I didn’t even try to build Rust from sources recently. I hope to work on it in the coming weeks. Sorry, I’m not of great help here :-/