Clean up the usage of V8_TARGET_ARCH_${arch} and V8_HOST_ARCH_${arch}

src/heap.h

 // 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 1937 matching lines @@
 
   // For keeping track of context disposals.
   int contexts_disposed_;
 
   int global_ic_age_;
 
   bool flush_monomorphic_ics_;
 
   int scan_on_scavenge_pages_;
 
-#if defined(V8_TARGET_ARCH_X64)
+#if V8_TARGET_ARCH_X64
   static const int kMaxObjectSizeInNewSpace = 1024*KB;
 #else
   static const int kMaxObjectSizeInNewSpace = 512*KB;
 #endif
 
   NewSpace new_space_;
   OldSpace* old_pointer_space_;
   OldSpace* old_data_space_;
   OldSpace* code_space_;
   MapSpace* map_space_;
@@ skipping 1105 matching lines @@
   DisallowHeapAllocation no_allocation;  // i.e. no gc allowed.
 
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(PathTracer);
 };
 #endif  // DEBUG
 
 } }  // namespace v8::internal
 
 #endif  // V8_HEAP_H_