[tlbuild] xz rebuild

[Karl Berry]

Hi TL builders. There's just one or two more pending changes before I'll
ask for a general rebuild, hopefully later today or tomorrow.

Meanwhile, this year it seems to be worth updating xz, since 5.6.0
can speed up decompression 2-3x and compression even more.

Please configure with --disable-shared to avoid shared library
dependencies; at least I hope that will suffice. Run ldd src/xz/xz
(or whatever utility) to check. Since nothing in TL can be done without
a working xz, please do confirm.

All TL needs is the xz binary, none of the other programs.
Feel free to commit yourself in Master/tlpkg/installer/xz (xz.PLATFORM)
or send the stripped and gzipped binary to me. (Email is fine, or a url,
whatever's easiest.)

Thanks,
Karl