Bigint tests fail on architectures other than x86_64 and i386 #6698

After updating from 3.3.0 to 3.4.0, I encountered consistent test failures on other CPU architectures (https://copr.fedorainfracloud.org/coprs/mkoncek/mruby/build/9987571/).
For a specific log, see for example here at the bottom: https://download.copr.fedorainfracloud.org/results/mkoncek/mruby/fedora-rawhide-s390x/09987571-mruby/builder-live.log.gz

I haven't delved into it. I will gladly help with finding the issue.