Add option to redirect stdout and stdcerr to a file for xcrun compatibility

Summary:
XCRun doesn't provide a convenient way to gather logs or know when a program has completed execution. (This is recognized by the dev community: see https://github.com/ios-control/ios-deploy/issues/588 )

We'll solve this by redirecting `stdout` and `stderr` to a file called `/tmp/BENCH_LOG`. Once the benchmark is complete, we'll create an empty file called `/tmp/BENCH_DONE` that we can poll for (let's say, every 5 seconds) to check whether the benchmark has completed.

Reviewed By: Jack-Khuu

Differential Revision: D53255873

fbshipit-source-id: cbc6581c42c58214ebdfb646069314e501150d91