[tlbuild] tl19 trial builds

[Karl Berry]

    Everything fine at -r50139 on aarch64-linux here.

Thanks Johannes, and everyone else.

    When shall we start to hand in the binaries?

Seems like tomorrow should be fine. (There have been a couple other
small changes, though doubtless they won't be the last.)

Dick said that x86_64-darwin is ready.

Norbert/Mojca, what's the status of the autobuild setup, e.g., for
x86_64-linux? And Mojca, what about x86_64-darwinlegacy?

I think all others not heard from are optional ... --thanks, karl.