skip CacheLocality optimizations if there are offline CPUs
skip CacheLocality optimizations if there are offline CPUs

Summary:
CacheLocality's parsing code for /proc/cpuinfo doesn't properly
handle systems in which there are offline CPUs. This diff makes it so
we fall back onto the sequential cache locality implementation in the
problematic scenarios.

This should address https://github.com/facebook/folly/issues/1208

Reviewed By: shixiao

Differential Revision: D17093176

fbshipit-source-id: 4da3a9dc53622f829af37ed70a2531c66fd92ddb

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly