Add a uwp-universal target to build all uwp arches at once

This is identical to how the cross-android-universal target works. Supported UWP arches are: x86, x86_64, arm64. This covers 100% of Windows 10 machines: https://docs.microsoft.com/en-us/windows/msix/package/device-architecture

Advantages are the same as Android:

1. Most people will build apps that target multiple (or all) archs
2. Easier to build and ship all arches at once
3. Easier to download two tarballs rather than six

Also updated the README to mention it, and output a single tarball with both devel and runtime when building for UWP. Usage:

./cerbero-uninstalled -c config/cross-uwp-universal.cbc package gstreamer-1.0