This patch introduces new Mac Catalyst support for mono!

Background Mac Catalyst allows a developer to use the traditionally iOS APIs - like UIKit - on macOS. It retains the same ABI as macOS x86_64, however, is compiled with a new -target flag that is required in all static and dynamic libraries.

This patch is my first port of mono so I appreciate any code reviews. :-)

I started with a clean build target so as not to break current iOS and Mac support. There is the potential to merge this with the existing iOS support, but I think this method of a specific maccat.mk file is a cleaner separation.

I discussed this publicly on the Merge Conflict podcast: https://www.mergeconflict.fm/225

I also show off the work on my Twitch stream: https://twitch.tv/FrankKrueger

This is the build script I've been using:

#!/bin/bash

export XCODE_DIR=/Applications/Xcode_12.0.0.app/Contents/Developer
export MACOS_VERSION=10.15

echo "ENABLE_MACCAT=1" > sdks/Make.config
make -C sdks/builds configure-maccat

make -j20 -C sdks/builds build-maccat

make -C sdks/builds archive-maccat

Backport of #20537.

/cc @akoeplinger @praeclarum