Bind(c): signed char is not a Fortran character type

Bind(c): signed char is not a Fortran character type

CFI_allocate and CFI_select_part were incorrectly treating
CFI_type_signed_char as a Fortran character type for the purpose of
deciding whether or not to use the elem_len argument.  It is a Fortran
integer type per table 18.2 in the 2018 Fortran standard.

Other functions in ISO_Fortran_binding.c appeared to handle this case
correctly already.

2021-07-15  Sandra Loosemore  <sandra@codesourcery.com>

libgfortran/
	* runtime/ISO_Fortran_binding.c (CFI_allocate): Don't use elem_len
	for CFI_type_signed_char.
	(CFI_select_part): Likewise.