Disable NNPACK build on unsupported CPU architectures
Disable NNPACK build on unsupported CPU architectures

Summary:
Few people complained in NNPACK repo about broken build on PPC64, as it specifically whitelists supported architecture in its CMakeLists.txt, and refuses to build on unsupported platforms. This commit explicitly disables NNPACK build (as part of Caffe2 build) on unsupported architectures.
Closes https://github.com/caffe2/caffe2/pull/1439

Differential Revision: D6288999

Pulled By: Maratyszcza

fbshipit-source-id: 76c40e9ce882356944b63968df8fd853f21ecd35

xzturn/caffe2,xzturn/caffe2,davinwang/caffe2,sf-wind/caffe2,Yangqing/caffe2,xzturn/caffe2,davinwang/caffe2,pietern/caffe2,sf-wind/caffe2,pietern/caffe2,davinwang/caffe2,pietern/caffe2,sf-wind/caffe2,caffe2/caffe2,pietern/caffe2,Yangqing/caffe2,xzturn/caffe2,xzturn/caffe2,Yangqing/caffe2,pietern/caffe2,davinwang/caffe2,Yangqing/caffe2,sf-wind/caffe2,sf-wind/caffe2,Yangqing/caffe2,davinwang/caffe2