release: use xz via pipe when compressing the tarballs.

libarchive(3) doesn't support the new liblzma API yet, but this change
allows us to enable multi-threaded xz compression.
``make release'' should now finish in half the time on a machine with
several cores and fast disks (our typical build server).

This behaviour only applies when building a release and it doesn't
affect buildworld/installworld. To disable threaded xz compression,
set XZ_THREADS=1.

Reviewed by: gjb
Tested by: gjb