mkinitcpio: use as many threads as possible (-T0) for xz compression
xz 5.4 changed the behaviour of -T0
to always use "multithreaded mode",
even on single-core CPUs.
See https://git.tukaani.org/?p=xz.git;a=commitdiff;h=0adc13bfe32c14f3e4c6ce9f2d4fdf4112ab53f4
This means that xz output when using -T0
should now be reproducible.