[tlbuild] another xz

[Karl Berry]

While we are waiting for some final source tweaks, there is another 
release of xz.  I updated Build/source/extra/xz and
http://tug.org/texlive/build.html#xz.

As I just wrote there, I'd like to ask everyone a favor: please rename
your binaries to xz{,dec}.<tlplatformname> before sending them to me.
It turns out to be rather a pain to do this x15 :).
FWIW, you can use tlmgr -print-arch to get the name if that's useful.

Thanks,
karl