characters sometimes failed on ARM
this was reported by Hermann Stamm-Wilbrandt: https://sympa.inria.fr/sympa/arc/cado-nfs/2023-07/msg00004.html
Files to reproduce are available here: https://stamm-wilbrandt.de/en/forum/cado.4x9eajs3.tar.gz
I can reproduce on gcc117, but the issue is non-deterministic (sometimes it works).
See https://sympa.inria.fr/sympa/arc/cado-nfs/2023-07/msg00015.html
See branch bug_characters_arm to investigate.