PHPCS supports reading from stdin.

Summary: PHPCS does actually support reading data from stdin, so let's make `ArcanistExternalLinter` aware of this.

Test Plan: `arc unit`.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9034