Fortran: Revert to non-multilib-specific ISO_Fortran_binding.h

Fortran: Revert to non-multilib-specific ISO_Fortran_binding.h

Commit fef67987cf502fe322e92ddce22eea7ac46b4d75 changed the
libgfortran build process to generate multilib-specific versions of
ISO_Fortran_binding.h from a template, by running gfortran to identify
the values of the Fortran kind constants C_LONG_DOUBLE, C_FLOAT128,
and C_INT128_T.  This caused multiple problems with search paths, both
for build-tree testing and installed-tree use, not all of which have
been fixed.

This patch reverts to a non-multilib-specific .h file that uses GCC's
predefined preprocessor symbols to detect the supported types and map
them to kind values in the same way as the Fortran front end.

2021-09-06  Sandra Loosemore  <sandra@codesourcery.com>

libgfortran/
	* ISO_Fortran_binding-1-tmpl.h: Deleted.
	* ISO_Fortran_binding-2-tmpl.h: Deleted.
	* ISO_Fortran_binding-3-tmpl.h: Deleted.
	* ISO_Fortran_binding.h: New file to replace the above.
	* Makefile.am (gfor_cdir): Remove MULTISUBDIR.
	(ISO_Fortran_binding.h): Simplify to just copy the file.
	* Makefile.in: Regenerated.
	* mk-kinds-h.sh: Revert pieces no longer needed for
	ISO_Fortran_binding.h.