Update config.sub to latest version.
# Configuration validation subroutine script.
# Copyright 1992-2021 Free Software Foundation, Inc.
-timestamp='2021-01-08'
+timestamp='2021-03-10'
# This file is free software; you can redistribute it and/or modify it
# under the terms of the GNU General Public License as published by
@@ -1712,7 +1712,7 @@ case $os in
| bosx* | nextstep* | cxux* | aout* | elf* | oabi* \
| ptx* | coff* | ecoff* | winnt* | domain* | vsta* \
| udi* | lites* | ieee* | go32* | aux* | hcos* \
- | chorusrdb* | cegcc* | glidix* \
+ | chorusrdb* | cegcc* | glidix* | serenity* \
| cygwin* | msys* | pe* | moss* | proelf* | rtems* \
| midipix* | mingw32* | mingw64* | mint* \
| uxpv* | beos* | mpeix* | udk* | moxiebox* \
