XNNPACK Pin Update

[mcr229]

🚀 The feature, motivation and pitch

For 0.7 release we want to update our XNNPACK Pin. There have been some build changes that require us to modify ExecuTorch, but this update should allow us to leverage improvements we've upstreamed like the new KleidiAI kernels. Some steps to facilitate the pin update:

• Update pthreadpool --> google/pthreadpool
• Deprecate SDPA operator
• modify target names from target --> xnnpack_target
• refactor build_src definitions in XNNPACK, to hook into our buck build system