This patch enables multi-threaded xz compression in the repacking stage
of building source tarballs.

With it applied, I get a ~2.2x speedup building the source of
linux-libre with 4 threads, compared to 1 thread.

Specifically, with the upstream linux-libre tarball in the page cache,
`guix build -S linux-libre` took ~366 seconds using a single thread for
xz, and ~164 seconds using 4 threads.