libfortran: Add script to regenerate source files

libfortran: Add script to regenerate source files

Move the regeneration of libgfortran source files from the Makefile
to a standalone script. We were previously highjacking the
maintainer mode of autoconf/automake in a way that was not adequate,
leading to various issues. Bring our behavior in line with other
GCC libraries.

2025-06-09  François-Xavier Coudert  <fxcoudert@gcc.gnu.org>

libgfortran/ChangeLog:

	PR libfortran/116400
	* Makefile.am: Remove source file regeneration rules.
	* Makefile.in: Regenerate.
	* regenerate.sh: New file.