lib/systems: Prohibit "gnu" ABI (*-gnu) with 32-bit ARM

It is ambiguous, and therefore banned within GCC.

lib/systems/default.nix

@@ -50,5 +50,12 @@ rec {
     } // mapAttrs (n: v: v final.parsed) inspect.predicates
       // args;
   in assert final.useAndroidPrebuilt -> final.isAndroid;
+     assert lib.foldl
+       (pass: { assertion, message }:
+         if assertion final
+         then pass
+         else throw message)
+       true
+       (final.parsed.abi.assertions or []);
     final;
 }