Introduce DropUnderReturnAddress for x64 port.

src/x64/macro-assembler-x64.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 822 matching lines @@
   // Load a heap object and handle the case of new-space objects by
   // indirecting via a global cell.
   void MoveHeapObject(Register result, Handle<Object> object);
 
   // Load a global cell into a register.
   void LoadGlobalCell(Register dst, Handle<Cell> cell);
 
   // 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);
   void Push(const Operand& src);
   void Push(Immediate value);
   void PushImm32(int32_t imm32);
   void Pop(Register dst);
   void Pop(const Operand& dst);
   void PushReturnAddressFrom(Register src) { pushq(src); }
   void PopReturnAddressTo(Register dst) { popq(dst); }
@@ skipping 771 matching lines @@
     masm->popfq();                                                           \
   }                                                                          \
   masm->
 #else
 #define ACCESS_MASM(masm) masm->
 #endif
 
 } }  // namespace v8::internal
 
 #endif  // V8_X64_MACRO_ASSEMBLER_X64_H_