X64 implementation of native ascii string compare.

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 fa7d33b1b2c061ee51cf04ddadb910d3cae475f1..60933ccfadb1341498965f09f32b8350a0630007 100644
--- a/src/x64/assembler-x64.h
+++ b/src/x64/assembler-x64.h
@@ -916,6 +916,10 @@ class Assembler : public Malloced {
     arithmetic_op_32(0x2B, dst, src);
   }
 
+  void subl(Register dst, const Operand& src) {
+    arithmetic_op_32(0x2B, dst, src);
+  }
+
   void subl(const Operand& dst, Immediate src) {
     immediate_arithmetic_op_32(0x5, dst, src);
   }