Move selective_build.bzl to shim_et (pytorch#14406)

This PR was created by the merge bot to help merge the original PR into
the main branch.
ghstack PR number: pytorch#14405 by
@kimishpatel
^ Please use this as the source of truth for the PR details, comments,
and reviews
ghstack PR base:
https://github.com/pytorch/executorch/tree/gh/kimishpatel/198/base
ghstack PR head:
https://github.com/pytorch/executorch/tree/gh/kimishpatel/198/head
Merge bot PR base: https://github.com/pytorch/executorch/tree/main
Merge bot PR head:
https://github.com/pytorch/executorch/tree/gh/kimishpatel/198/orig
@diff-train-skip-merge

Co-authored-by: Kimish Patel <[email protected]>

Author: pytorchbot