s390x CI builds time-out when running tests

[gnzlbg]

CI for s390x-unknown-linux-gnu only tests that stdsimd builds properly. If NORUN=1 is disabled, the tests time-out (10 minutes without output) while running (everything builds properly under docker though).

It might well be that we need to use qemu-system instead of qemu-user to test s390x, or that we might have to choose a different cpu (IIRC s390x targets a z10 so the generic qemu one should work fine though).

cc @malbarbo