contrib: Add autoregen.py

This script is a copy of the current script used by Sourceware's
autoregen buildbots.

It is intended as a helper to regenerate files managed by autotools
(autoconf, automake, aclocal, ....), as well as the toplevel
Makefile.in which is created by autogen.

Other files can be updated when using maintainer-mode, but this is not
covered by this script.

2024-04-19  Christophe Lyon  <christophe.lyon@linaro.org>

	contrib/
	* autoregen.py: New script.