I created the AUR package armutils. It contains forks of mkarchroot, arch-nspawn and makechrootpkg that are adjusted for creating ARM chroots / building ARM packages on x86_64 hosts. The source code is on GitLab. It's in an early stage. Therefore, it would be great if you could try it out and give feedback.
Since I didn't find a solution for this problem, I created a fork of makepkg that uses sudo instead of fakeroot. That fork is actually used to build packages.
Thanks, mipi