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.