target/ppc/cpu_init: make cpu listing deterministic

The existing compare function only make sure that we list cpus with PVR
order. However, we never compare cpu names.
As a result, while cpus are grouped per PVR, the order within a group is
non deterministic. Depending on QOM type initialization order, we get
different results for -cpu help.

For instance, previous output could be:
Available CPUs:
  755_v1.0         PVR 00083100
  745_v1.0         PVR 00083100
  755_v1.1         PVR 00083101
  745_v1.1         PVR 00083101

While a sorted output should be:
Available CPUs:
  745_v1.0         PVR 00083100
  755_v1.0         PVR 00083100
  745_v1.1         PVR 00083101
  755_v1.1         PVR 00083101

Fix it by comparing cpu names to have a stable result.
This allows us to record and compare various command line results across
versions to make sure we didn't break anything while working on
single-binary.

Signed-off-by: Pierrick Bouvier <pierrick.bouvier@oss.qualcomm.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@oss.qualcomm.com>
Tested-by: Philippe Mathieu-Daudé <philmd@oss.qualcomm.com>
Reviewed-by: Amit Machhiwal <amachhiw@linux.ibm.com>
Tested-by: Amit Machhiwal <amachhiw@linux.ibm.com>
Link: https://lore.kernel.org/qemu-devel/20260720192403.66694-1-pierrick.bouvier@oss.qualcomm.com
Signed-off-by: Harsh Prateek Bora <harshpb@linux.ibm.com>