Allow for /proc/cpuinfo files lacking "cpu cores" and/or "siblings" entries.

Borrow code from localeModels/gbt/maxTaskPar to handle the case where
/proc/cpuinfo doesn't have the entries we expect for hyperthreads.
Merge pull request #668 from gbtitus/proc-cpuinfo-no-cores-sibs

Allow for /proc/cpuinfo files lacking "cpu cores" and/or "siblings" entries.