update build flags for benchmark binaries
Summary:
Suggested by Shoaib Meenai, we should use mode/ndk_libcxx to replace mode/gnustl.
This diff updated all build flags for caffe2 and pytorch in aibench. For easy management, I created two mode files in xplat/caffe2/mode, and delete buckconfig.ptmobile.pep.
Test Plan:
caffe2
```
buck run aibench:run_bench -- -b aibench/specifications/models/caffe2/squeezenet/squeezenet.json --remote --devices s9f
```
https://our.intern.facebook.com/intern/aibench/details/433604719423848
full jit
```
buck run aibench:run_bench -- -b aibench/specifications/models/pytorch/fbnet/fbnet_mobile_inference.json --platform android/full_jit --framework pytorch --remote --devices SM-G960F-8.0.0-26
```
https://our.intern.facebook.com/intern/aibench/details/189359776958060
lite interpreter
```
buck run aibench:run_bench -- -b aibench/specifications/models/pytorch/fbnet/fbnet_mobile_inference.json --platform android --framework pytorch --remote --devices s9f
```
https://our.intern.facebook.com/intern/aibench/details/568178969092066
Reviewed By: smeenai
Differential Revision: D23338089
fbshipit-source-id: 62f4ae2beb004ceaab1f73f4de8ff9e0c152d5ee
