Enable building ETDump with CMake (#564)

Summary: Pull Request resolved: https://github.com/pytorch/executorch/pull/564

Test Plan:
```
cd executorch
mkdir cmake-out && cd cmake-out
cmake -DEXECUTORCH_BUILD_SDK=1 ..
cd ..
cmake --build cmake-out -j9 -t etdump
```

Will add CI in a follow up diff.

Reviewed By: Jack-Khuu

Differential Revision: D49834286

Pulled By: tarun292

fbshipit-source-id: 66dbe48662c8d8f84ca28c7aec26a5f11706e2b7