Build binary packages for Qt for Webassembly

For the tech preview in Qt 5.11, we will make source packages of Qt for WebAssembly, but if we want to have this as a fully supported platform in Qt 5.12, we need the CI to produce binary artifacts, and these need to be packages and made part of the unified installer like other platforms.

The information we have about Qt for WebAssembly is here: https://wiki.qt.io/Qt_for_WebAssembly

The provisioning needs to be set up to download specified packages of Emscripten and set up the build environment.

See here: https://kripken.github.io/emscripten-site/docs/getting_started/downloads.html

In the instructions, there is a git repository that is checked out, but this is mainly for getting the scripts to get the packages and set up the environment. We can look at those to find the right URLs and environment variables that need to be set. After that, it is merely a case of configuring Qt to build for WebAssembly.