Arm Baremetal Runner cleanup (#1909)

Summary:
This converts the arm_executor_runner into a examples/arm/ standalone cmake project which depends on the Ethos-U sdk and the Executorch runtime for bare metal, rather than being a series of patches.

This cleans up all but a few issues, most prominent remaining are:
 * removing the final patch against the sdk linker script (WIP)
 * linking against the CMAKE executorch project cleanly rather than importing individual libraries (there isn't currently a CMAKE target for third party projects outside executorch repo)

The next anticipated changes would be to remove the remaining linker script patch and vela patch so we are working with clean checkouts of the respective projects.

Pull Request resolved: https://github.com/pytorch/executorch/pull/1909

Reviewed By: JacobSzwejbka

Differential Revision: D53811799

Pulled By: digantdesai

fbshipit-source-id: 95bc7f73dfaf7d8b649b50702e73c422c9a8190d