[tlbuild] auxiliary binaries for pretest

[Karl Berry]

Thanks to everyone for building the new binaries so quickly.

I guess what's left, for the moment, are the auxiliary binaries, for
those platforms that need them (and haven't already sent to me):

  Current release of xz:
  https://tukaani.org/xz/xz-5.2.5.tar.xz

  Current release of lz4:
  https://github.com/lz4/lz4/releases/tag/v1.9.3

  Current release of wget:
  http://ftpmirror.gnu.org/wget/wget-1.21.tar.gz
  
http://tug.org/texlive/build.html#xz has our attempts at recording build
instructions. Improvements welcome as ever.

Again, lz4 is purely optional (used only for faster compression of
backups), so don't feel compelled to provide it.

(Norbert, for various x86-linux I guess you built xz and lz4 on jessie
last ... ?)

Also, FYI, I updated the asy sources, but purely with portability
patches, so no need to rebuild. --thanks, karl.

(Sorry, I don't have any good way to know version numbers.)

aarch64-linux: xz
amd64-freebsd: wget xz
amd64-netbsd: wget xz
armhf-linux: lz4 xz
win32: lz4.exe wget.exe xz.exe
i386-cygwin: lz4 xz
i386-freebsd: wget xz
i386-linux: lz4 xz
i386-netbsd: wget xz
i386-solaris: lz4 wget xz
universal-darwin: lz4 wget xz
x86_64-cygwin: lz4 xz
x86_64-darwinlegacy: lz4 wget xz
x86_64-linux: lz4 xz
x86_64-linuxmusl: lz4 wget xz
x86_64-solaris: lz4 wget xz