[VMR] Crossgen and ILC are disabled in VMR cross-arch builds

[jtschuster]

In cross-arch builds, the VMR doesn't have a valid runtime pack for the build machine. The in-build ilc and crossgen require a runtime pack to be able to run on packages in the build. Since there is no runtime pack, they fail at runtime. We work around this checking for Crossbuild=true and DotnetBuildVertical=true and disabling ilc and crossgen in those cases. We should get a valid host runtime pack as a prerequisite and reenable ilc and crossgen.

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