libgfortran: Fix up LIBGFOR_CHECK_FLOAT128 [PR106137]

libgfortran: Fix up LIBGFOR_CHECK_FLOAT128 [PR106137]

My recent gfortran + libgfortran patch apparently broke (some?) aarch64
builds.  While it is desirable to use just _Float128 rather than __float128,
we only want to use it (and e.g. define HAVE_FLOAT128) on targets where
_Float128 is supported and long double isn't IEEE quad precision.
Which is targets that support __float128 type which we have been testing
for before - _Float128 is supported on those targets and on targets where
long double is IEEE quad precision.

So, the following patch restores check for whether __float128 is supported
into the LIBGFOR_CHECK_FLOAT128 check which determines whether
HAVE_FLOAT128 is defined or whether to use libquadmath, so that e.g. on
aarch64 where long double is IEEE quad we don't do that.

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

	PR bootstrap/106137
	* acinclude.m4 (LIBGFOR_CHECK_FLOAT128): Adjust comment.
	Also test for __float128.
	(HAVE_FLOAT128): Adjust description.
	* config.h.in: Regenerated.
	* configure: Regenerated.