[tlbuild] x86_64-linux builder

[Karl Berry]

For x86_64-linux builds of TeX Live, we seem to go through a repetitive
process of several people volunteering, the build being done on too new
of a system for some users, then someone else tries a build, repeat.

So, how about this: anyone who can build on x86_64-linux, please just
say what kernel+libc version you can build on (and anything else
relevant).  Then maybe we can get clear on this from the start.

Thanks,
Karl