The ISA of the CPU cores doesn't matter. The PCIe controller does. And Broadcom being Broadcom, they have a weird one with interesting bugs like 64-bit I/O not even working.
On something like a MACCHIATObin or Honeycomb LX2K, AMD GPUs just work, even with pre-boot display in UEFI (thanks to EDK2 optionally including QEMU for this, it can just emulate x86 for the display driver it reads from the card). Well, the MCbin has a fun quirk with device enumeration, but actual operation of the device is perfect.
