(maint) Fix Linux physicalprocessorcount fact to handle missing physical id.

If a processor doesn't report a "physical id" in /proc/cpuinfo, then we
were counting it as a logical processor but not as a physical processor.

This fix corrects the issue by counting the processor in the set of cpus
given the processor id instead of the physical id.  There will be no
overlap between the two identifiers since it's not possible for one
processor to have a physical id and another to be missing a physical
id.  That would imply that it's a multi-processor system with a CPU that
isn't in a socket.