Fortran: Use _Float128 rather than __float128 for c_float128 kind.

The GNU Fortran manual documents that the c_float128 kind corresponds
to __float128, but in fact the implementation uses float128_type_node,
which is _Float128.  Both refer to the 128-bit IEEE/ISO encoding, but
some targets including aarch64 only define _Float128 and not __float128,
and do not provide quadmath.h.  This caused errors in some test cases
referring to __float128.

This patch changes the documentation (including code comments) and
test cases to use _Float128 to match the implementation.

2021-09-16  Sandra Loosemore  <sandra@codesourcery.com>

gcc/fortran/

	* intrinsic.texi (ISO_C_BINDING): Change C_FLOAT128 to correspond
	to _Float128 rather than __float128.
	* iso-c-binding.def (c_float128): Update comments.
	* trans-intrinsic.c (gfc_builtin_decl_for_float_kind): Likewise.
	(build_round_expr): Likewise.
	(gfc_build_intrinsic_lib_fndcecls): Likewise.
	* trans-types.h (gfc_real16_is_float128): Likewise.

gcc/testsuite/
	* gfortran.dg/PR100914.c: Do not include quadmath.h.  Use
	_Float128 _Complex instead of __complex128.
	* gfortran.dg/PR100914.f90: Add -Wno-pedantic to suppress error
	about use of _Float128.
	* gfortran.dg/c-interop/typecodes-array-float128-c.c: Use
	_Float128 instead of __float128.
	* gfortran.dg/c-interop/typecodes-sanity-c.c: Likewise.
	* gfortran.dg/c-interop/typecodes-scalar-float128-c.c: Likewise.
	* lib/target-supports.exp
	(check_effective_target_fortran_real_c_float128): Update comments.

libgfortran/
	* ISO_Fortran_binding.h: Update comments.
	* runtime/ISO_Fortran_binding.c: Likewise.