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.