maintainer-scripts/gcc_release: compress xz in parallel

maintainer-scripts/gcc_release: compress xz in parallel

1. This should speed up decompression for folks, as parallel xz
   creates a different archive which can be decompressed in parallel.

   Note that this different method is enabled by default in a new
   xz release coming shortly anyway (>= 5.3.3_alpha1).

   I build GCC regularly from the weekly snapshots
   and so the decompression time adds up.

2. It should speed up compression on the webserver a bit.

   Note that -T0 won't be the default in the new xz release,
   only the parallel compression mode (which enables parallel
   decompression).

   -T0 detects the number of cores available.

   So, if a different number of threads is preferred, it's fine
   to set e.g. -T2, etc.

Signed-off-by: Sam James <sam@gentoo.org>
	* gcc_release (XZ): Add -T0.