Rename TARGET_OS_* to HOST_OS_*.

runtime/bin/extensions_fuchsia.cc

 // Copyright (c) 2016, 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 "platform/globals.h"
-#if defined(TARGET_OS_FUCHSIA)
+#if defined(HOST_OS_FUCHSIA)
 
 #include "bin/extensions.h"
 #include <dlfcn.h>  // NOLINT
 
 #include "platform/assert.h"
 
 namespace dart {
 namespace bin {
 
 const char* kVmSnapshotDataSymbolName = "_kDartVmSnapshotData";
@@ skipping 24 matching lines @@
   const char* err_str = dlerror();
   if (err_str != NULL) {
     return Dart_NewApiError(err_str);
   }
   return Dart_Null();
 }
 
 }  // namespace bin
 }  // namespace dart
 
-#endif  // defined(TARGET_OS_FUCHSIA)
+#endif  // defined(HOST_OS_FUCHSIA)