Fix typo.

src/platform-win32.cc

 // Copyright 2012 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 1341 matching lines @@
   // Read the symbols.
   if (!LoadSymbols(process_handle)) return kStackWalkError;
 
   // Capture current context.
   CONTEXT context;
   RtlCaptureContext(&context);
 
   // Initialize the stack walking
   STACKFRAME64 stack_frame;
   memset(&stack_frame, 0, sizeof(stack_frame));
-#if V8_OS _WIN64
+#if V8_OS_WIN64
   stack_frame.AddrPC.Offset = context.Rip;
   stack_frame.AddrFrame.Offset = context.Rbp;
   stack_frame.AddrStack.Offset = context.Rsp;
 #else
   stack_frame.AddrPC.Offset = context.Eip;
   stack_frame.AddrFrame.Offset = context.Ebp;
   stack_frame.AddrStack.Offset = context.Esp;
 #endif
   stack_frame.AddrPC.Mode = AddrModeFlat;
   stack_frame.AddrFrame.Mode = AddrModeFlat;
@@ skipping 615 matching lines @@
   limit_mutex = CreateMutex();
 }
 
 
 void OS::TearDown() {
   delete limit_mutex;
 }
 
 
 } }  // namespace v8::internal