Fix setup module to gather facts from PowerPC Macs.

Based on email thread from ansible-devel. This patch continues to work as before for newer Intel Macs, but falls back to an alternate way of determining processor and processor_cores for older PowerPC Macs.

I've tested against my PowerMac G5 with these facts returned:
```
"ansible_architecture": "Power Macintosh",
"ansible_distribution": "MacOSX", 
"ansible_distribution_version": "10.5.8", 
"ansible_model": "PowerMac7,2",
"ansible_processor": "PowerPC 970 (2.2) @ 2 GHz",
"ansible_processor_cores": "2",
```