Rename TARGET_OS_* to HOST_OS_*.

runtime/vm/thread_interrupter_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 "vm/thread_interrupter.h"
 
 #include "platform/assert.h"
 
 namespace dart {
 
 bool ThreadInterrupter::IsDebuggerAttached() {
   return false;
 }
 
 void ThreadInterrupter::InterruptThread(OSThread* thread) {
   UNIMPLEMENTED();
 }
 
 
 void ThreadInterrupter::InstallSignalHandler() {
   UNIMPLEMENTED();
 }
 
 
 void ThreadInterrupter::RemoveSignalHandler() {
   UNIMPLEMENTED();
 }
 
 }  // namespace dart
 
-#endif  // defined(TARGET_OS_FUCHSIA)
+#endif  // defined(HOST_OS_FUCHSIA)