fix -Wunused-but-set-variable for gcc-4.6 on x64

src/x64/regexp-macro-assembler-x64.cc

 // Copyright 2011 the V8 project authors. 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.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 643 matching lines @@
     return true;
   }
   case 'w': {
     if (mode_ != ASCII) {
       // Table is 128 entries, so all ASCII characters can be tested.
       __ cmpl(current_character(), Immediate('z'));
       BranchOrBacktrack(above, on_no_match);
     }
     __ movq(rbx, ExternalReference::re_word_character_map());
     ASSERT_EQ(0, word_character_map[0]);  // Character '\0' is not a word char.
-    ExternalReference word_map = ExternalReference::re_word_character_map();
     __ testb(Operand(rbx, current_character(), times_1, 0),
              current_character());
     BranchOrBacktrack(zero, on_no_match);
     return true;
   }
   case 'W': {
     Label done;
     if (mode_ != ASCII) {
       // Table is 128 entries, so all ASCII characters can be tested.
       __ cmpl(current_character(), Immediate('z'));
       __ j(above, &done);
     }
     __ movq(rbx, ExternalReference::re_word_character_map());
     ASSERT_EQ(0, word_character_map[0]);  // Character '\0' is not a word char.
-    ExternalReference word_map = ExternalReference::re_word_character_map();
     __ testb(Operand(rbx, current_character(), times_1, 0),
              current_character());
     BranchOrBacktrack(not_zero, on_no_match);
     if (mode_ != ASCII) {
       __ bind(&done);
     }
     return true;
   }
 
   case '*':
@@ skipping 699 matching lines @@
   }
 }
 
 #undef __
 
 #endif  // V8_INTERPRETED_REGEXP
 
 }}  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64