[tlbuild] PPC builds

[Karl Berry]

   mm> saying: "if you are still
   mm> using these binaries, please leave us feedback; ...

Maybe so. I don't much want to insert such a thing right now, though,
would rather do it after the release if we do it. The current regime of
removing platforms on a more or less ad hoc basis doesn't seem like it's
caused too many problems.

    jh> https://www.tug.org/texlive/build.html (that has to be revised anyway).

I update it after the release, not before :). -k