Ported SubStringStub to X64.

src/x64/assembler-x64.h

Index: src/x64/assembler-x64.h
diff --git a/src/x64/assembler-x64.h b/src/x64/assembler-x64.h
index 1bddb2fb03d235b8de757c495535412b91238d05..522098a7322f9b054e5f7cdc0800fa9c09eb3911 100644
--- a/src/x64/assembler-x64.h
+++ b/src/x64/assembler-x64.h
@@ -574,6 +574,13 @@ class Assembler : public Malloced {
   void movzxwq(Register dst, const Operand& src);
   void movzxwl(Register dst, const Operand& src);
 
+  // Repeated moves.
+
+  void repmovsb();
+  void repmovsw();
+  void repmovsl();
+  void repmovsq();
+
   // New x64 instruction to load from an immediate 64-bit pointer into RAX.
   void load_rax(void* ptr, RelocInfo::Mode rmode);
   void load_rax(ExternalReference ext);