[tlbuild] trial TL20 builds

[Karl Berry]

Luigi has committed an update to LuaTeX that's intended to be
essentially what we release for TL20. I think all the major intended
updates are in now (including asymptote). So we are in the period where
we should try to build and hopefully find and fix compilation failures.
So, please give it a try.

If all goes well, perhaps we can switch to the pretest sooner than
expected. I wrote down Feb 28 for that date, but the sooner we get the
new binaries in people's hands, the better. --thanks, karl.

P.S. Johannes, I will try to make the POWER changes you suggest
tomorrow/soon. Doesn't sound too painful.