Separate benchmark implementation to executor_runner_lib for sdk/fb/executor_runner (#1208)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1208

Separate out benchmark functionality into `:executor_runner_lib` target so the benchmark implementation can be included in other targets without causing duplicate symbol errors (i.e. `int main()`).

Part of a series of diffs to enable running executor_runner on iOS

Reviewed By: kirklandsign

Differential Revision: D51013791

fbshipit-source-id: dd4fb47093fa77735916c4f256d67729a0ac8db4