Support frames of builtins when stack walking.

Author: fangerer

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/nodes/frame/ReadCallerFrameNode.java

@@ -162,6 +162,10 @@ public Frame visitFrame(FrameInstance frameInstance) {
                                 // avoid overriding the location if we don't know it
                                 if (callNode != null) {
                                     info.setCallNode(callNode);
+                                } else if (pRootNode.isInternal()) {
+                                    // builtin functions may call without a Truffle call node; in
+                                    // this case, we use the root node as location
+                                    info.setCallNode(pRootNode);
                                 }
                                 // We may never return a frame without location because then we
                                 // cannot materialize it.