Adds Runtime call stub and Dart entry stub to ARM64.

runtime/vm/assembler_x64.h

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef VM_ASSEMBLER_X64_H_
 #define VM_ASSEMBLER_X64_H_
 
 #ifndef VM_ASSEMBLER_H_
 #error Do not include assembler_x64.h directly; use assembler.h instead.
 #endif
@@ skipping 647 matching lines @@
   /*
    * Macros for High-level operations and implemented on all architectures.
    */
 
   void CompareRegisters(Register a, Register b);
 
   // Issues a move instruction if 'to' is not the same as 'from'.
   void MoveRegister(Register to, Register from);
   void PopRegister(Register r);
 
+  // Macros accepting a pp Register argument may attempt to load values from
+  // the object pool when possible. Unless you are sure that the untagged object
+  // pool pointer is in another register, or that it is not available at all,
+  // PP should be passed for pp.
   void AddImmediate(Register reg, const Immediate& imm, Register pp);
   void AddImmediate(const Address& address, const Immediate& imm, Register pp);
 
   void Drop(intptr_t stack_elements);
 
   enum Patchability {
     kPatchable,
     kNotPatchable,
   };
 
@@ skipping 382 matching lines @@
 }
 
 
 inline void Assembler::EmitOperandSizeOverride() {
   EmitUint8(0x66);
 }
 
 }  // namespace dart
 
 #endif  // VM_ASSEMBLER_X64_H_