Rename TARGET_OS_* to HOST_OS_*.

runtime/bin/directory_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/directory.h"
 
 #include <dirent.h>    // NOLINT
 #include <errno.h>     // NOLINT
 #include <stdlib.h>    // NOLINT
 #include <string.h>    // NOLINT
 #include <sys/param.h>  // NOLINT
 #include <sys/stat.h>  // NOLINT
 #include <unistd.h>    // NOLINT
@@ skipping 407 matching lines @@
   ExistsResult exists = Exists(path);
   if (exists != EXISTS) {
     return false;
   }
   return (NO_RETRY_EXPECTED(rename(path, new_path)) == 0);
 }
 
 }  // namespace bin
 }  // namespace dart
 
-#endif  // defined(TARGET_OS_FUCHSIA)
+#endif  // defined(HOST_OS_FUCHSIA)