SIMD ABI change warnings (g++-8)

dune-common/dune/common/simd/loop.hh: In function ‘auto Dune::Simd::Overloads::cond(Dune::Simd::Overloads::ADLTag<5, (std::is_same<bool, typename Dune::Simd::Overloads::ScalarType<typename std::decay<_Tp>::type>::type>::value && (lanes<M>() == lanes<Dune::LoopSIMD<T, S, A> >()))>, M, Dune::LoopSIMD<T, S, A>, Dune::LoopSIMD<T, S, A>) [with M = Dune::LoopSIMD<bool, 5, 64>; T = long double; long unsigned int S = 5; long unsigned int A = 64]’:
dune-common/dune/common/simd/loop.hh:346:12: note: The ABI for passing parameters with 64-byte alignment has changed in GCC 4.6
       auto cond(ADLTag<5, std::is_same<bool, Simd::Scalar<M> >::value
            ^~~~