Import xz-embedded from git. This is from commit hash '6a8a2364434763a033781f6b2a605ace9a021013'. This makes it possible to use CRC64 but for now it's intentionally not added to build.
Approved by: adrian (mentor)
The upcoming geom_compress module (a read-only gzip/ulzma translation layer, similar to what geom_uzip does) will leverage parts of this.