Enable XNNPACK ops on iOS and macOS.
Test Plan: buck run aibench:run_bench -- -b aibench/specifications/models/pytorch/pytext/pytext_mobile_inference.json --platform ios --framework pytorch --remote --devices D221 (https://github.com/pytorch/pytorch/commit/9788a74da8fdba0675f0e67a0fbe55c3eb5dc486)AP-12.0.1
Reviewed By: xta0
Differential Revision: D21886736
fbshipit-source-id: ac482619dc1b41a110a3c4c79cc0339e5555edeb
