Do not export INTERFACE_LINK_LIBRARIES from non-linkable targets
When cmExportFileGenerator::SetImportLinkInterface exports the old LINK_INTERFACE_LIBRARIES property values it skips doing so for non-linkable targets because target->GetLinkInterface returns NULL for such targets. Since cmExportFileGenerator::PopulateInterfaceLinkLibrariesProperty looks at the INTERFACE_LINK_LIBRARIES property directly instead of using the computed link interface, teach it to skip exporting the property if target->IsLinkable returns false. Extend the RunCMake.CMP0022 test with a case covering this. Simply export an executable target that links to a library that is not exported.