libgfortran: Switch some more __float128 uses to _Float128

My patch apparently left some __float128 uses in libgfortran
that could use _Float128 instead, the following patch changes that.

2022-06-29  Jakub Jelinek  <jakub@redhat.com>

	* mk-kinds-h.sh: Change __float128 to _Float128 in a comment.
	* acinclude.m4 (LIBGFOR_CHECK_MATH_IEEE128): Use _Float128 instead of
	__float128.
	* libgfortran.h (isnan): Change __float128 to _Float128 in a comment.
	(__acoshieee128, __acosieee128, __asinhieee128, __asinieee128,
	__atan2ieee128, __atanhieee128, __atanieee128, __copysignieee128,
	__coshieee128, __cosieee128, __erfcieee128, __erfieee128,
	__expieee128, __fabsieee128, __fmaieee128, __fmodieee128, __jnieee128,
	__log10ieee128, __logieee128, __powieee128, __sinhieee128,
	__sinieee128, __sqrtieee128, __tanhieee128, __tanieee128,
	__ynieee128, __strtoieee128): Use _Float128 instead of __float128.
	* configure: Regenerated.