test failure on i386 (simd stuff)

Some tests fail on i386 (without SSE), for example:

/<<PKGBUILDDIR>>/dune/common/simd/test.hh:1100: In checkBinaryOpVS, while testing Dune::Simd::UnitTest::OpInfixDiv (Dune::AlignedNumberImpl::AlignedNumber<double, 32u>&, double&): Error: check (lane(l, vopres) == static_cast<T>(op(lane(l, static_cast<V1>(vref1)), static_cast<T2>(sref2) ))) failed

See also the full build log

Adding -ffloat-store to the compiler flags works around the problem, so one side of the comparison probably wasn't stored to memory and has still extra precision in the register. I'll do that for the Debian packages for now.

@joe wrote the tests, but I'm not sure how much time to invest in i386. I don't currently have time to spend on this. !387 (merged) fixed similar issues (and the number of the merge request is probably the best part of this!)