Use rep_movsb for copying one-byte strings in substring intrinsic.

runtime/vm/assembler_x64.cc

Index: runtime/vm/assembler_x64.cc
===================================================================
--- runtime/vm/assembler_x64.cc	(revision 21290)
+++ runtime/vm/assembler_x64.cc	(working copy)
@@ -390,6 +390,13 @@
 }
 
 
+void Assembler::rep_movsb() {
+  AssemblerBuffer::EnsureCapacity ensured(&buffer_);
+  EmitUint8(0xF3);
+  EmitUint8(0xA4);
+}
+
+
 void Assembler::leaq(Register dst, const Address& src) {
   AssemblerBuffer::EnsureCapacity ensured(&buffer_);
   EmitOperandREX(dst, src, REX_W);