Introduce DropUnderReturnAddress for x64 port.

src/x64/macro-assembler-x64.cc

Index: src/x64/macro-assembler-x64.cc
diff --git a/src/x64/macro-assembler-x64.cc b/src/x64/macro-assembler-x64.cc
index 1fd7c2c94624a1a7e0b8b5426dbbf18c44d146ad..a5c4c5615986437f3b3d60eb4d99a00668531bdc 100644
--- a/src/x64/macro-assembler-x64.cc
+++ b/src/x64/macro-assembler-x64.cc
@@ -2666,6 +2666,20 @@ void MacroAssembler::Drop(int stack_elements) {
 }
 
 
+void MacroAssembler::DropUnderReturnAddress(int stack_elements,
+                                            Register scratch) {
+  ASSERT(stack_elements > 0);
+  if (kPointerSize == kInt64Size && stack_elements == 1) {
+    popq(MemOperand(rsp, 0));
+    return;
+  }
+
+  PopReturnAddressTo(scratch);
+  Drop(stack_elements);
+  PushReturnAddressFrom(scratch);
+}
+
+
 void MacroAssembler::Push(Register src) {
   if (kPointerSize == kInt64Size) {
     pushq(src);