Build support for s390x: installers and packages (#53950)

Build support for s390x: installers and packages (#53950)

* Introduce "PrimaryRuntimeFlavor" concept, set to Mono for s390x

* Enable installers, packages, and archives when building the
  PrimaryRuntimeFlavor, even when this is Mono instead of CoreCLR

* Update default subsets in eng/Subsets.props to take
  PrimaryRuntimeFlavor into account

* Skip packaging singlefilehost when not available (due to building
  with the Mono runtime flavor)

* Only build the IL tools packages in coreclr-packages.proj
  when building with the Mono runtime flavor