testsuite: Fix up ext-floating2.C on powerpc64-linux

testsuite: Fix up ext-floating2.C on powerpc64-linux

Another testcase that is failing on powerpc64-linux.  The test expects
a diagnostics when float64 && float128 or in another spot when
float32 && float128.  Now, float128 effective target is satisfied on
powerpc64-linux, despite __CPP_FLOAT128_T__ not being defined, because
one needs to add some extra options for it.  I think 32-bit arm has
similar case for float16.

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

	* g++.dg/cpp23/ext-floating2.C: Add dg-add-options for
	float16, float32, float64 and float128.