Bind(C): Fix type encodings in ISO_Fortran_binding.h

Bind(C): Fix type encodings in ISO_Fortran_binding.h

ISO_Fortran_binding.h had many incorrect hardwired kind encodings in
the definitions of the CFI_type_* macros.  Additionally, not all
targets support all the defined type encodings, and the Fortran
standard requires those macros to have a negative value.

This patch changes ISO_Fortran_binding.h to use sizeof instead of
hard-coded sizes, and assembles it from fragments that reflect the
set of types supported by the target.

2021-07-22  Sandra Loosemore  <sandra@codesourcery.com>
	    Tobias Burnus  <tobias@codesourcery.com>

libgfortran/
	PR libfortran/101305
	* ISO_Fortran_binding.h: Fix hard-coded sizes and split into...
	* ISO_Fortran_binding-1-tmpl.h: New file.
	* ISO_Fortran_binding-2-tmpl.h: New file.
	* ISO_Fortran_binding-3-tmpl.h: New file.
	* Makefile.am: Add rule for generating ISO_Fortran_binding.h.
	Adjust pathnames to that file.
	* Makefile.in: Regenerated.
	* mk-kinds-h.sh: New file.
	* runtime/ISO_Fortran_binding.c: Fix include path.