Use radix_mmu environment variable to select between Hash or Radix MMU, when performing the CAS method call.

This matches kernel's behavior, by selecting Hash MMU by default and Radix if radix_mmu is not zero, to make sure that both loader and kernel always select the same MMU.

If the detected CPU is not a POWER9, then Hash MMU is selected, regardless of radix_mmu value, as in kernel.

Tested on QEMU pseries with KVM, selecting both Hash and Radix MMUs.