Mostly remove "STDERR" and "STDOUT" constants from Arcanist

Summary:
Ref T13675. Ref T13556. The "STDOUT" and "STDERR" constants are defined by the PHP CLI SAPI, in cli_register_file_handles().

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.

Subscribers: cspeckmim

Maniphest Tasks: T13675, T13556

Differential Revision: https://secure.phabricator.com/D21794