Fix arch eval error introduced in #61019

This occurs when using a `platform.gcc.arch` that isn't one of the
pre-existing hard-coded options.

(cherry picked from commit 8f3efbde4e)

lib/systems/architectures.nix

@@ -60,7 +60,7 @@ rec {
   };
 
   predicates = let
-    featureSupport = feature: x: builtins.elem feature features.${x};
+    featureSupport = feature: x: builtins.elem feature features.${x} or [];
   in {
     sse3Support    = featureSupport "sse3";
     ssse3Support   = featureSupport "ssse3";