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

@@ -205,7 +205,7 @@ def command_qemu(
                 ]
             )
         ]
-        command += [chroot_native / "usr/bin" / f"qemu-system-{arch}"]
+        command += [chroot_native / "usr/bin" / f"qemu-system-{arch.qemu()}"]
         command += ["-L", chroot_native / "usr/share/qemu/"]
 
     command += ["-nodefaults"]