Make __float128 use the _Float128 type.

Make __float128 use the _Float128 type.

2022-08-12   Michael Meissner  <meissner@linux.ibm.com>

gcc/

	* config/rs6000/rs6000-builtin.cc (rs6000_init_builtins): Always use the
	_Float128 type for __float128.  Don't use the long double type.
	* config/rs6000/rs6000.cc (rs6000_libgcc_floating_mode_supported_p):
	Use KFmode for Float128 mode.
	(rs6000_floatn_mode): Likewise.
	(rs6000_c_mode_for_suffix): Likewise.