fs/common.mk: enable multithreaded xz compression

fs/common.mk: enable multithreaded xz compression

xz help indicates only 1 thread is used unless we set threads:
-T, --threads=NUM   use at most NUM threads; the default is 1; set to 0
                    to use as many threads as there are processor cores

Since this splits the file into blocks, the result will be not
bit-for-bit identical to single-threaded compression. Therefore, don't
enable this in BR2_REPRODUCIBLE builds.

Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
Reviewed-by: Matthew Weber <matthew.weber@rockwellcollins.com>
[Arnout: append the option instead of repeating the entire command]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>