Mostly remove "STDERR" and "STDOUT" constants from Arcanist
The "native arc" embedded PHP wrapper doesn't define these, and there's no real reason to define them, since they're just defined in terms of the PHP stream wrappers ("php://stdin", etc) anyway.
This patch isn't exhaustive (and a subsequent change should add lint, rejecting these magic constants) but is just trying to make native arc functional.
Test Plan: Created this revision with a standalone native arc binary.
Differential Revision: https://secure.phabricator.com/D21794