Rename TARGET_OS_* to HOST_OS_*.

runtime/vm/native_symbol_macos.cc

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/globals.h"
-#if defined(TARGET_OS_MACOS)
+#if defined(HOST_OS_MACOS)
 
 #include "vm/native_symbol.h"
 
 #include <cxxabi.h>  // NOLINT
 #include <dlfcn.h>   // NOLINT
 
 namespace dart {
 
 void NativeSymbolResolver::InitOnce() {}
 
@@ skipping 22 matching lines @@
 }
 
 
 void NativeSymbolResolver::FreeSymbolName(char* name) {
   free(name);
 }
 
 
 }  // namespace dart
 
-#endif  // defined(TARGET_OS_MACOS)
+#endif  // defined(HOST_OS_MACOS)