* [registrar] Make a copy of the static registrar for Xamarin.Mac/Classic.
Make a copy of the static registrar for Xamarin.Mac/Classic, one which is
compatible with the binary artifacts we ship for Xamarin.Mac/Classic
(libxammac, XamMac.dll).
This way the static registrar can evolve in the future, without breaking
Xamarin.Mac/Classic.
* [runtime] Make a copy of the runtime headers too.
* Update comment.
* [mmp] Delay creating a Target instance until we know if we're a Classic or Unified app.
Otherwise the wrong static registrar might be created.
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