[tlbuild] 2020 pretest builds

[Karl Berry]

I'd like to start updating the binaries for the TL 2020 pretest,
including asy. I'm going to update the support files shortly.

Dick already provided x86_64-darwin and Mojca and Norbert's automated
builds should be fine, and I expect to commit those soon. Anyone else,
please send me urls or commit, as you like.

I believe there are also new versions of xz, lz4, and wget, but it's not
important to update those right away. --thanks, karl.