Rename TARGET_OS_* to HOST_OS_*.

runtime/bin/log_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/log.h"
 
 #include <stdio.h>  // NOLINT
 
 namespace dart {
 namespace bin {
 
 void Log::VPrint(const char* format, va_list args) {
   vfprintf(stdout, format, args);
   fflush(stdout);
 }
 
 void Log::VPrintErr(const char* format, va_list args) {
   vfprintf(stderr, format, args);
   fflush(stderr);
 }
 
 }  // namespace bin
 }  // namespace dart
 
-#endif  // defined(TARGET_OS_FUCHSIA)
+#endif  // defined(HOST_OS_FUCHSIA)