Link cpuinfo when using external NNPACK
Link cpuinfo when using external NNPACK

Summary:
Close #1685
Closes https://github.com/caffe2/caffe2/pull/1722

Differential Revision: D6686071

Pulled By: Maratyszcza

fbshipit-source-id: bbe86bfd479376bc7cdfdd0bad3896f1c2356216

Yangqing/caffe2,Yangqing/caffe2,xzturn/caffe2,xzturn/caffe2,Yangqing/caffe2,xzturn/caffe2,xzturn/caffe2,Yangqing/caffe2,Yangqing/caffe2,xzturn/caffe2,caffe2/caffe2