Move xnnpack taget to fb code base (#88909)
1. Move the source file list to the `build_variables.bzl`, as it's the source of truth for both internal buck build and oss build
2. Move target definitions to `fb` internal folder
3. Some changes are triggered from auto format.
Differential Revision: [D40906961](https://our.internmc.facebook.com/intern/diff/D40906961/)
**NOTE FOR REVIEWERS**: This PR has internal Meta-specific changes or comments, please review them on [Phabricator](https://our.internmc.facebook.com/intern/diff/D40906961/)!
Pull Request resolved: https://github.com/pytorch/pytorch/pull/88909
Approved by: https://github.com/mcr229
