Same problem here with a RPi3, completely broken after the kernel update.
What worked for me:
1. extract uSD card and plug into another linux machine
2. mount temporary file systems
3. mount boot partition
4. chroot to the uSD card (this requires qemu-arm-static, there are many guides online)
5. downgrade the kernel to a previous version (which you will find in pacman's cache). I am currently back to 5.4.35-1 but I think any version is fine
6. unmount everything
7. uSD card back into RPi, boot, everything fine
Doing $this->bbcode_second_pass_code('', 'sudo pacman -Syu') still proposes the update to linux-raspberrypi-5.10.73-1 that broke everything, but maybe this time I will wait
Well, at least I learned some new things...