Add SNPE deps for caffe2 benchmark android binary
Summary:
Adding snpe dependencies to caffe2_benchmark so that this can benchmark SNPE models on portal devices.
Also need to change ndk_libcxx to gnustl till snpe is updated to work with ndk.
Test Plan: Tested on top of the stack.
Reviewed By: linbinyu
Differential Revision: D23569397
fbshipit-source-id: a6281832804ed4fbb5a8406f436caeae1ff4fd2b
