Fix GCC 4.7 warnings, which are related to char being signed in GCC

src/mksnapshot.cc

 // Copyright 2006-2008 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 91 matching lines @@
   virtual int Position() { return data_.length(); }
   void Print(FILE* fp) {
     int length = Position();
     for (int j = 0; j < length; j++) {
       if ((j & 0x1f) == 0x1f) {
         fprintf(fp, "\n");
       }
       if (j != 0) {
         fprintf(fp, ",");
       }
-      fprintf(fp, "%d", at(j));
+      fprintf(fp, "%u", static_cast<unsigned char>(at(j)));
     }
   }
   char at(int i) { return data_[i]; }
   bool Compress(Compressor* compressor) {
     ASSERT_EQ(-1, raw_size_);
     raw_size_ = data_.length();
     if (!compressor->Compress(data_.ToVector())) return false;
     data_.Clear();
     data_.AddAll(*compressor->output());
     return true;
@@ skipping 216 matching lines @@
   sink.WriteSpaceUsed(
       partial_ser.CurrentAllocationAddress(i::NEW_SPACE),
       partial_ser.CurrentAllocationAddress(i::OLD_POINTER_SPACE),
       partial_ser.CurrentAllocationAddress(i::OLD_DATA_SPACE),
       partial_ser.CurrentAllocationAddress(i::CODE_SPACE),
       partial_ser.CurrentAllocationAddress(i::MAP_SPACE),
       partial_ser.CurrentAllocationAddress(i::CELL_SPACE),
       partial_ser.CurrentAllocationAddress(i::LO_SPACE));
   return 0;
 }