Rename TARGET_OS_* to HOST_OS_*.

runtime/vm/virtual_memory_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 "vm/globals.h"
-#if defined(TARGET_OS_FUCHSIA)
+#if defined(HOST_OS_FUCHSIA)
 
 #include "vm/virtual_memory.h"
 
 #include <magenta/process.h>
 #include <magenta/status.h>
 #include <magenta/syscalls.h>
 #include <sys/mman.h>
 #include <unistd.h>
 
 #include "platform/assert.h"
@@ skipping 256 matching lines @@
             end_address - page_address, prot, mx_status_get_string(status));
     return false;
   }
   LOG_INFO("mx_vmar_protect(%lx, %lx, %x) success\n", page_address,
            end_address - page_address, prot);
   return true;
 }
 
 }  // namespace dart
 
-#endif  // defined(TARGET_OS_FUCHSIA)
+#endif  // defined(HOST_OS_FUCHSIA)