Add example script to run on cpu (#611)

Summary:
An example script to build and run executor_runner baremetal version on ARM M-class CPUs

```
cd executorch
rm -rf cmake-out # optional
cd examples/arm/
./setup.sh [scratch_dir]
./run.sh [scratch_dir]
```

Pull Request resolved: https://github.com/pytorch/executorch/pull/611

Reviewed By: larryliu0820

Differential Revision: D49900956

Pulled By: digantdesai

fbshipit-source-id: eba5cf2e3680c231cb602e1252203f7095fa5b17