Add tzcnt as an acceptable instruction to x86-64 validator (it already

src/trusted/validator/x86/decoder/gen/ncopcode_insts_impl.h

 /* native_client/src/trusted/validator/x86/decoder/gen/ncopcode_insts_impl.h
  * THIS FILE IS AUTO_GENERATED DO NOT EDIT.
  *
  * This file was auto-generated by enum_gen.py
  * from file ncopcode_insts.enum
  */
 
 /* Define the corresponding names of NaClMnemonic. */
 static const char* const g_NaClMnemonicName[NaClMnemonicEnumSize + 1] = {
   "Aaa",
@@ skipping 585 matching lines @@
   "Subpd",
   "Subps",
   "Subsd",
   "Subss",
   "Swapgs",
   "Syscall",
   "Sysenter",
   "Sysexit",
   "Sysret",
   "Test",
+  "Tzcnt",
   "Ucomisd",
   "Ucomiss",
   "Ud2",
   "Unpckhpd",
   "Unpckhps",
   "Unpcklpd",
   "Unpcklps",
   "Verr",
   "Verw",
   "Vmload",
   "Vmmcall",
   "Vmrun",
   "Vmsave",
   "Wbinvd",
   "Wrmsr",
   "Xadd",
   "Xchg",
   "Xlat",
   "Xor",
   "Xorpd",
   "Xorps",
   "NaClMnemonicEnumSize"
 };
 
 const char* NaClMnemonicName(NaClMnemonic name) {
   return name <= NaClMnemonicEnumSize
     ? g_NaClMnemonicName[name]
     : "NaClMnemonic???";
 }