The test FAILs on 32-bit targets that don't have __int128 type.

2020-03-12  Jakub Jelinek  <jakub@redhat.com>

	PR c++/93907
	* g++.dg/cpp2a/concepts-using2.C (cc): Use long long instead of
	__int128 if __SIZEOF_INT128__ isn't defined.