Rename TARGET_OS_* to HOST_OS_*.

runtime/bin/file_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/file.h"
 
 #include <errno.h>      // NOLINT
 #include <fcntl.h>      // NOLINT
 #include <libgen.h>     // NOLINT
 #include <sys/mman.h>   // NOLINT
 #include <sys/stat.h>   // NOLINT
 #include <sys/types.h>  // NOLINT
 #include <unistd.h>     // NOLINT
@@ skipping 559 matching lines @@
   }
   return ((file_1_info.st_ino == file_2_info.st_ino) &&
           (file_1_info.st_dev == file_2_info.st_dev))
              ? File::kIdentical
              : File::kDifferent;
 }
 
 }  // namespace bin
 }  // namespace dart
 
-#endif  // defined(TARGET_OS_FUCHSIA)
+#endif  // defined(HOST_OS_FUCHSIA)