testsuite: Fix up ext-floating15.C tests on powerpc64-linux [PR109278]

testsuite: Fix up ext-floating15.C tests on powerpc64-linux [PR109278]

I've noticed this test FAILs on powerpc64-linux, with
FAIL: g++.dg/cpp23/ext-floating15.C  -std=gnu++98 (test for excess errors)
Excess errors:
/home/jakub/gcc/gcc/testsuite/g++.dg/cpp23/ext-floating15.C:8:5: error: '_Float128' is not supported on this target
/home/jakub/gcc/gcc/testsuite/g++.dg/cpp23/ext-floating15.C:8:5: error: '_Float128' is not supported on this target
/home/jakub/gcc/gcc/testsuite/g++.dg/cpp23/ext-floating15.C:8:1: error: variable or field 'bar' declared void
/home/jakub/gcc/gcc/testsuite/g++.dg/cpp23/ext-floating15.C:8:5: error: '_Float128' is not supported on this target
/home/jakub/gcc/gcc/testsuite/g++.dg/cpp23/ext-floating15.C:8:6: error: expected primary-expression before '_Float128'
and similarly other std versions.
powerpc64-linux is float128 target, but needs to add some options for it.

Fixed by adding them.

2023-04-25  Jakub Jelinek  <jakub@redhat.com>

	PR c++/109278
	* g++.dg/cpp23/ext-floating15.C: Add dg-add-options float128.