Add a --target flag to arc unit for uploading results

Summary: Ref T5821. Basic idea here is that Harbormaster can run arc unit --everything --target ... to get a build target updated.

Test Plan: Ran arc unit --target ..., saw web UI update with test results.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T5821

Differential Revision: https://secure.phabricator.com/D14190