Rename TARGET_OS_* to HOST_OS_*.

runtime/bin/platform_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/platform.h"
 
 #include <string.h>  // NOLINT
 #include <unistd.h>  // NOLINT
 
 #include "bin/dartutils.h"
 #include "bin/fdutils.h"
 #include "bin/file.h"
 
@@ skipping 97 matching lines @@
 }
 
 
 void Platform::Exit(int exit_code) {
   exit(exit_code);
 }
 
 }  // namespace bin
 }  // namespace dart
 
-#endif  // defined(TARGET_OS_FUCHSIA)
+#endif  // defined(HOST_OS_FUCHSIA)