Currently there is no working widevine version for stock archlinux ARM available, see this corresponding github issue: https://github.com/xbmc/inputstream.adaptive/issues/678.
This has also been discussed in this this post: https://archlinuxarm.org/forum/viewtopic.php?f=60&t=15376 (ARMv7h board).
There is a request for an AUR package: https://bbs.archlinux.org/viewtopic.php?id=270689. But I doubt this request will be heard there, because it's an ARM only topic. Maybe this is something more fitting for an archlinuxarm specific package?
What's the opinion of the experts here? I would create a PR if there is some advice where to locate such a package.
Thanks for your feedback,
Peter