Use pre-built mpy-cross if appropriate

[jepler]

Pre-built mpy cross are available and cover the majority of systems where you can imagine wanting to run it:

2020-08-28T18:51:05.000Z        780.2 kB       mpy-cross.static-x64-windows-6.0.0-alpha.3.exe
2020-08-28T18:51:04.000Z        1.6 MB         mpy-cross.static-amd64-linux-6.0.0-alpha.3
2020-08-28T18:51:03.000Z        1007.2 kB      mpy-cross.static-raspbian-6.0.0-alpha.3
2020-08-28T18:50:45.000Z        281.2 kB       mpy-cross-macos-catalina-6.0.0-alpha.3

Right now, the 6.0.0-alpha.3 mpy-cross can also be used for 5.3, though it's a slightly off-label use.

Downloading this binary instead of building mpy-cross from source would speed things up, and avoid people needing to install a whole C compiler toolchain (which is easy enough on mac and linux but a tough experience on Windows)