mono: Update msbuild to 16.3, support for git branches

To get WebAssembly templates to actually work, we need to use
Mono 6.6 (currently in preview), which doesn't have tags yet
but is in the `2019-08` branch. So I dehardcoded the `mono-`
prefix for the git tree-ish to allow using `./build.sh 2019-08`.

6.6 doesn't build with msbuild 16.0, so I'm upgrading to its
matching version 16.3. There are no DEB files for 16.3 on the
EOL'ed Ubuntu 14.04, but it seems that the 16.04 packages install
cleanly.

We also backport a patch from mono's master branch to fix wasm
support further, and enable parallel build jobs for make.

Dockerfile.android

@@ -16,9 +16,10 @@ RUN if [ -z "${mono_version}" ]; then echo -e "\n\nargument mono-version is mand
 ENV ANDROID_HOME=/root/sdk/
 ENV ANDROID_NDK_ROOT=/root/sdk/ndk-bundle/
 
-RUN git clone https://github.com/mono/mono --branch mono-${mono_version} --single-branch && \
+RUN git clone https://github.com/mono/mono --branch ${mono_version} --single-branch && \
     cd mono && git submodule update --init && cd .. && \
     export MONO_SOURCE_ROOT=/root/mono && \
+    export make="make -j" && \
     git clone https://github.com/godotengine/godot-mono-builds && \
     cd godot-mono-builds && \
     git checkout bd129da22b8b9c96f3e8b07af348cc5fb61504bf && \