[cortex-m] initial commit (#10381)

This PR was created by the merge bot to help merge the original PR into
the main branch.
ghstack PR number: https://github.com/pytorch/executorch/pull/10265 by
@digantdesai
^ 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/digantdesai/31/base
ghstack PR head:
https://github.com/pytorch/executorch/tree/gh/digantdesai/31/head
Merge bot PR base: https://github.com/pytorch/executorch/tree/main
Merge bot PR head:
https://github.com/pytorch/executorch/tree/gh/digantdesai/31/orig
@diff-train-skip-merge

Co-authored-by: Digant Desai <digantdesai@meta.com>