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;
 }

lib/systems/parse.nix

@@ -192,7 +192,15 @@ rec {
   abis = setTypes types.openAbi {
     android = {};
     cygnus = {};
-    gnu = {};
+    gnu = {
+      assertions = [
+        { assertion = platform: !platform.isAarch32;
+          message = ''
+            The "gnu" ABI is ambiguous on 32-bit ARM. Use "gnueabi" or "gnueabihf" instead.
+          '';
+        }
+      ];
+    };
     msvc = {};
     eabi = {};
     androideabi = {};
@@ -270,7 +278,7 @@ rec {
       kernel = getKernel args.kernel;
       abi =
         /**/ if args ? abi       then getAbi args.abi
-        else if isLinux   parsed then abis.gnu
+        else if isLinux   parsed then (if isAarch32 parsed then abis.gnueabi else abis.gnu)
         else if isWindows parsed then abis.gnu
         else                     abis.unknown;
     };