SystemInformation: try harder to find a useful processor name

GetExtendedProcessorName() may sometimes return "Unknown family" when
we could return something more meaningful. Some systems (e.g. PA-RISC
on Linux) export a "cpu" value in /proc/cpuinfo that has a beautified
processor name in it (e.g. "PA8600 (PCX-W+)"). If we have not found
something nice but this field is present use that information.

Change-Id: I72561bdcba12f3f43d86f96bf3ebfae36ea22421