[5] move XNNPACK to shared BUCK build (#80209)
Summary: Reland D36529332 (https://github.com/pytorch/pytorch/commit/b8b46f932b41b5b5969d49fbe5ea8d2d5b45a1e3) with shared buck build structure.
Test Plan:
buck build //xplat/third-party/XNNPACK:XNNPACK
Sandcastle
Differential Revision: D37407609
Pull Request resolved: https://github.com/pytorch/pytorch/pull/80209
Approved by: https://github.com/kimishpatel
