Adds support for building Mono templates for Android in 3.2/master.
We use the script from https://github.com/godotengine/godot-mono-builds
to build Mono for all arches with the right options, and a patch included
from https://github.com/godotengine/godot/blob/master/modules/mono/build_scripts/patches/fix-mono-android-tkill.diff
which is also necessary.
I also moved the SDK to a `sdk` folder to make the `$HOME` more tidy,
especially since the Mono build scripts install the mono toolchains in
`$HOME` too by default.
Using a git clone for specific mono version tag, as the upstream tarballs
are iffy (the 5.18.1.3 tarball lacks files needed to build on Android).
* Mono config on MacOSX is not portable from the Linux and Windows
version.
* Rename the compilers so that 'gcc' and 'g++' are the new version by
default.
* Add default environment variables to containers.
Rémi Verschelde
Hein-Pieter van Braam-Stewart