libstdc++: Fix test that fails for targets without __int128 [PR 96042]

libstdc++: Fix test that fails for targets without __int128 [PR 96042]

When backporting this test (and the changes it depends on) I forgot that
the __max_diff_type and __max_size_type classes are only present on
trunk, not the gcc-10 branch. That using iota_view<long long, long long>
oonly works correctly when __int128 is available, so the test fails on
32-bit targets.

This just skips the failing check.

	PR libstdc++/96042
	* testsuite/std/ranges/iota/96042.cc: Only assert that the
	difference type is wider than long long if __int128 is
	supported.