Support ppc64le and fix cross architecture bootstrapping

This pull request adds support for ppc64le in pmbootstrap and fixes bootstrapping of a new architecture. The generated cross-compiler still needs the same fixups as pmaports@ad9a7874, but works fine and the QEMU target can be built successfully and booted (pending pmaports pull request with necessary changes, will open after this gets merged).

pmb/qemu/run.py

@@ -249,6 +249,9 @@ def command_qemu(
     elif arch == Arch.riscv64:
         command += ["-M", "virt"]
         command += ["-device", "virtio-gpu-pci"]
+    elif arch == Arch.ppc64le:
+        command += ["-M", "pseries"]
+        command += ["-device", "virtio-gpu-pci"]
     else:
         raise RuntimeError(f"Architecture {arch} not supported by this command" " yet.")