Rename TARGET_OS_* to HOST_OS_*.

runtime/vm/os_linux.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 "vm/globals.h"
-#if defined(TARGET_OS_LINUX)
+#if defined(HOST_OS_LINUX)
 
 #include "vm/os.h"
 
 #include <errno.h>         // NOLINT
 #include <limits.h>        // NOLINT
 #include <malloc.h>        // NOLINT
 #include <time.h>          // NOLINT
 #include <sys/resource.h>  // NOLINT
 #include <sys/time.h>      // NOLINT
 #include <sys/types.h>     // NOLINT
@@ skipping 404 matching lines @@
   abort();
 }
 
 
 void OS::Exit(int code) {
   exit(code);
 }
 
 }  // namespace dart
 
-#endif  // defined(TARGET_OS_LINUX)
+#endif  // defined(HOST_OS_LINUX)