Use config directives to carve out a 'portable' kernel stdlib.

Swift SVN r23202

stdlib/core/ArrayCast.swift

@@ -16,6 +16,10 @@
 //
 //===----------------------------------------------------------------------===//
 
+#if _runtime(_ObjC)
+// FIXME: These need to be implemented even for non-objc:
+// rdar://problem/18881196
+
 enum _ValueOrReference {
 case Reference, Value
   init<T>(_: T.Type) {
@@ -185,3 +189,4 @@ public func _arrayConditionalCast<SourceElement, TargetElement>(
     return _arrayConditionalBridgeElements(source)
   }
 }
+#endif