This reverts commit c00556e1.

Instead of cloning the mono-build-tools-extra and building the tools in
it, now get the binaries from Stevedore. These are only used on macOS
builds of Mono.

This repo has been moved to Github Enterprise, so point the clone code
in the build script to the new location.

Change the compiler used to build Mono for Android from gcc to clang,
and update the NDK to version r16b.

Also, remove some unused ARM architectures.

This change ensures that the unityjit and unityaot profiles match the
.NET 4.7.1 API by using the profile stubber to modify the IL code in the
class library assemblies.

This reverts commit 2b4caf8b.

This reverts commit 62189df9.

Our previous committed mono versions are too old to build latest mono
and we were falling back to monolite to build.