Adds Runtime call stub and Dart entry stub to ARM64.

runtime/vm/assembler_x64.h

Index: runtime/vm/assembler_x64.h
===================================================================
--- runtime/vm/assembler_x64.h	(revision 35189)
+++ runtime/vm/assembler_x64.h	(working copy)
@@ -665,6 +665,10 @@
   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);