Richard, currently the upstream s390x target does not compile without -disable-werror.
[...] In file included from REPOS/qemu/tcg/tcg.c:205:0: REPOS/qemu/tcg/s390/tcg-target.c:2231:13: warning: ‘sigill_handler’ defined but not used [-Wunused-function] [...] Seems that we can simply remove this. Can you have a look? Christian