Features: Extend the generalized_initializers test for GNU < 4.7.

That compiler requires a different initializer_list constructor, so
update the test to match.

Tests/CompileFeatures/cxx_generalized_initializers.cpp

@@ -8,6 +8,7 @@ namespace std {
     const _E* __begin_;
     size_t    __size_;
 
+    initializer_list(const int*, long unsigned int) {}
   };
 }