Not sure if I did it right but seems the kernel configs are identical, between -1 and the legacy -3 version. So maybe the naming itself is breaking something else in the system then.
edit: seems this issue happened before:
https://github.com/raspberrypi/linux/issues/503
way over what I can understand.
this was a strange mix of me just getting into Arch for ARM and testing it, the second day devs moving to new naming scheme and at the same time upstream kernel having this issue with gpu_mem.