Enable multi-thread support in xz compression
    
    xz-utils starting 5.2.0 (2014-12-21) support multi-threaded compression.
    This version was added to Fedora 22 (2015-01-07) and also available in
    CentOS 7.
    
    -T 0 allows xz to use all available cores on the system assuming input
    file is big enough and there is enough memory on the system.
    
    Tested on SiFive HiFive Unleashed (RISC-V, riscv64) with Fedora and it
    showed significant time savings.
    
    Signed-off-by: David Abdurachmanov <david.abdurachmanov@gmail.com>