fix: the global gap report keeps standalone files

With no platform named, cross_reference passed no standalone cores, so every dual profile was laid out as libretro only. The CLI and the site's gaps page left out 133 files that Batocera and RetroBat run standalone and that no platform declares (eka2l1 90, dolphin 21, cemu 11). Both now pass the union of every platform's standalone cores.

scripts/generate_site.py

@@ -618,9 +618,14 @@ def build_emulator_gap_report(
         for k, v in profiles.items()
         if v.get("type") not in ("alias", "test")
     }
+    from common import standalone_set
+
     return run_cross_reference(
         unique_profiles, declared, db,
         data_names=data_names, all_declared=all_declared,
+        standalone_cores=set().union(
+            *(standalone_set(cov["config"]) for cov in coverages.values())
+        ),
     )