it used to be:
ulimit -c unlimited
to enable the core dump to be written.
But since archlinux switched to systemd stuff has changed.
A fairly easy way to revert to the old behavior:
ln -s /dev/null /etc/sysctl.d/50-coredump.conf
and reboot
more about it in a forum post over at archlinux
now you can just set ulimit -c unlimited again and have fun debugging