Introduce DropUnderReturnAddress for x64 port.

src/x64/macro-assembler-x64.h

Index: src/x64/macro-assembler-x64.h
diff --git a/src/x64/macro-assembler-x64.h b/src/x64/macro-assembler-x64.h
index a2163642d5c0e841248df843eac860d8c8f7a5d0..8fd31fc6e39fef43b3aca2a2ceea756637dfcf71 100644
--- a/src/x64/macro-assembler-x64.h
+++ b/src/x64/macro-assembler-x64.h
@@ -840,6 +840,11 @@ class MacroAssembler: public Assembler {
   // Emit code to discard a non-negative number of pointer-sized elements
   // from the stack, clobbering only the rsp register.
   void Drop(int stack_elements);
+  // Emit code to discard a positive number of pointer-sized elements
+  // from the stack under the return address which remains on the top,
+  // clobbering the rsp register.
+  void DropUnderReturnAddress(int stack_elements,
+                              Register scratch = kScratchRegister);
 
   void Call(Label* target) { call(target); }
   void Push(Register src);