Need OS-specific `mscorlib.dll`/etc. builds

[jonpryor]

As part of continuing corefx/etc. code merges into Mono, mscorlib.dll and System.Data.dll (and others?) are going to be OS-specific. As such, we will need to rebuild the BCL for each supported platform (Android, macOS, Windows), instead of using the same binary assemblies on all platforms.

[jonpryor]

How do we build?

We need to change our configure invocations for each OS to not disable the BCL build.
Then, between each OS build, we need to rename mono/mcs/class/lib/monodroid so that when we rebuild the BCL we don't nuke the previous output.

Mono currently uses deterministic builds, so after building the BCL for all assemblies, we can just diff(1) them to see if they're OS-specific or not. If they're identical, don't copy to the OS-specific install dir (below). If they're unique, copy them.

Install Structure?

xbuild-frameworks/MonoAndroid/v1.0 will continue to be for Android-OS assemblies.

Platform-specific mscorlib.dll/etc. will be placed into xbuild/Xamarin/Android/host-$(OSName), e.g. xbuild/Xamarin/Android/lib/host-Darwin/mscorlib.dll.

Integrations?

We'll need to allow the Android Designer to "know" where the OS-specific files are, so that they're copied in preference to the default Android-OS files.

[jonpryor]

This was resolved in commit d5e002e.