[tlbuild] asymptote 2.87 rebuild

[Karl Berry]

I committed the sources for asy 2.87, and will update the runtime
shortly.

Those of you who build asy manually, please do, and commit or send me
the binaries as usual. All that's needed is the asy executable.
Please strip it.

For the CI builds ... Mojca, are you still building asy
somewhere/somehow, or can you easily restore your builds? I did not see
it in x86_64-linux, amd64-freebsd, x86_64-darwinlegacy at
https://build.contextgarden.net/dl/texlive. (I admit I didn't check the
others.) Help? --thanks, karl.