Crossgen2 support for System.Private.CoreLib compilation in CoreCLR build (#44090)

Crossgen2 support for System.Private.CoreLib compilation in CoreCLR build (#44090)

This change moves the logic for crossgenning System.Private.CoreLib to the crossgen-corelib.proj script, no longer calling out to the OS-specific scripts crossgen-corelib.cmd / crossgen-corelib.sh, and adds the support for using Crossgen2 for the System.Private.CoreLib compilation. As of this commit Crossgen2 is not set as the default because the System.Utf8.Experimental libraries tests fail with Crossgen2-compiled CoreLib. I'm working on fixing this as the next step.

Thanks

Tomas