add sys._framework field on darwin which is now used in site.py

Author: timfel

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/builtins/modules/SysModuleBuiltins.java

@@ -145,6 +145,9 @@ public void initialize(PythonCore core) {
 
         String os = getPythonOSName();
         builtinConstants.put("platform", os);
+        if (os.equals("darwin")) {
+            builtinConstants.put("_framework", PNone.NONE);
+        }
         builtinConstants.put("__gmultiarch", getPythonArch() + "-" + os);
 
         super.initialize(core);