[build Folder Migration] Move build/Codegen.cmake (again) (#9314)

### Summary
I know I moved this into `scripts/build` in
https://github.com/pytorch/executorch/pull/9185/, but I think
`tools/cmake` makes more sense. This will fit perfectly with
`tools/buck` (D71267118), etc.

### Test plan

CI

cc @larryliu0820 @lucylq