Rename TARGET_OS_* to HOST_OS_*.

runtime/vm/os_thread_macos.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"  // NOLINT
-#if defined(TARGET_OS_MACOS)
+#if defined(HOST_OS_MACOS)
 
 #include "vm/os_thread.h"
 
 #include <sys/errno.h>         // NOLINT
 #include <sys/types.h>         // NOLINT
 #include <sys/sysctl.h>        // NOLINT
 #include <mach/mach_init.h>    // NOLINT
 #include <mach/mach_host.h>    // NOLINT
 #include <mach/mach_port.h>    // NOLINT
 #include <mach/mach_traps.h>   // NOLINT
@@ skipping 412 matching lines @@
 
 void Monitor::NotifyAll() {
   // When running with assertions enabled we track the owner.
   ASSERT(IsOwnedByCurrentThread());
   int result = pthread_cond_broadcast(data_.cond());
   VALIDATE_PTHREAD_RESULT(result);
 }
 
 }  // namespace dart
 
-#endif  // defined(TARGET_OS_MACOS)
+#endif  // defined(HOST_OS_MACOS)