Implemented FastAsciiStringJoin in X64 full codegen.

src/x64/assembler-x64.h

 // Copyright (c) 1994-2006 Sun Microsystems Inc.
 // 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.
 //
@@ skipping 1119 matching lines @@
   void xor_(const Operand& dst, Immediate src) {
     immediate_arithmetic_op(0x6, dst, src);
   }
 
   // Bit operations.
   void bt(const Operand& dst, Register src);
   void bts(const Operand& dst, Register src);
 
   // Miscellaneous
   void clc();
+  void cld();
   void cpuid();
   void hlt();
   void int3();
   void nop();
   void nop(int n);
   void rdtsc();
   void ret(int imm16);
   void setcc(Condition cc, Register reg);
 
   // Label operations & relative jumps (PPUM Appendix D)
@@ skipping 457 matching lines @@
  private:
   Assembler* assembler_;
 #ifdef DEBUG
   int space_before_;
 #endif
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_X64_ASSEMBLER_X64_H_