Fortran: Fix c_float128 and c_float128_complex definitions.

Fortran: Fix c_float128 and c_float128_complex definitions.

gfc_float128_type_node is only non-NULL on targets that support a
128-bit type that is not long double.  Use float128_type_node instead
when computing the value of the kind constants c_float128 and
c_float128_complex from the ISO_C_BINDING intrinsic module; this also
ensures it actually corresponds to __float128 (the IEEE encoding) and
not some other 128-bit floating-point type.

2021-08-11  Sandra Loosemore  <sandra@codesourcery.com>

gcc/fortran/
	* iso-c-binding.def (c_float128, c_float128_complex): Check
	float128_type_node instead of gfc_float128_type_node.
	* trans-types.c (gfc_init_kinds, gfc_build_real_type):
	Update comments re supported 128-bit floating-point types.