Rename TARGET_OS_* to HOST_OS_*.

runtime/bin/fdutils_fuchsia.cc

 // Copyright (c) 2012, 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/fdutils.h"
 
 #include <errno.h>      // NOLINT
 #include <fcntl.h>      // NOLINT
 #include <sys/ioctl.h>  // NOLINT
 #include <unistd.h>     // NOLINT
 
 #include "platform/signal_blocker.h"
 
@@ skipping 123 matching lines @@
 
 void FDUtils::SaveErrorAndClose(intptr_t fd) {
   int err = errno;
   NO_RETRY_EXPECTED(close(fd));
   errno = err;
 }
 
 }  // namespace bin
 }  // namespace dart
 
-#endif  // defined(TARGET_OS_FUCHSIA)
+#endif  // defined(HOST_OS_FUCHSIA)