[tlbuild] new xz release

[Karl Berry]

There is a new version of xz: xz-4.999.9beta-124-gb637.
It's in the repository in Build/source/extra as before,
or you can download it from http://tukaani.org/xz.

Despite the painful version number, the changes are apparently about
robustness and portability, and the new release is "recommended".  The
compressed files should be interoperable with our previous version.

Please build it (tug.org/texlive/build.html#xz) and send me the stripped
xz and xzdec binaries (or commit them under Master/tlpkg/installer/xz --
not in the regular bin dirs).

If there are problems building, please see the bug reporting information
in the xz-*/README file.

Thanks,
Karl