[CI] Add an "OSX .NET Core" build.

[jpobst]

Adds a Mac - .NET Core build to Azure Pipelines.

Some notable changes:

• Adds the Prepare target on Mac (.NET Core only) to move closer to what Windows uses, with the eventual goal of all systems being identical.
• Adds the Microsoft.NETFramework.ReferenceAssemblies NuGet package to all assemblies that target net472. This allows them to be built on .NET Core, which does not have net472 framework assemblies.
• Fixed a lot of backslashes in <Exec> tasks. Mono will fix these for us, dotnet will not.
• Expanded jnienv-gen logic to handle running jnienv-gen.dll on dotnet in more places.

Note Mono is still used in the following cases:

• make prepare-core uses Mono/MSBuild to build BootstrapTasks.dll to run JdkInfo.props.
• mono jnienv-gen.exe is used when it is run from a project build targeting net472.
• Mono headers are needed to build libjava-interop.dylib.

Eventually these first 2 uses of mono could probably be removed, But this gets us started.