MFV: xz-embedded 3f438e15109229bb14ab45f285f4bff5412a9542

MFC after: 2 weeks

MFV r262756:

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.

Initial copy of xz-embedded to sys/contrib/.

The upcoming geom_compress module (a read-only gzip/ulzma translation layer,
similar to what geom_uzip does) will leverage parts of this.