[Wilds] Move ArcanistRuntime to support/ArcanistRuntime.php

Summary:
Ref T13098. Currently, it lives in init-script.php. Move it to a separate file for similarity with support/PhabricatorStartup.php.

Two small changes here:

• dirname(dirname(...)) adjustments for new path.
• Remove memory_limit(-1), this is adjusted by the libphutil init script already.

Test Plan: Ran arc.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13098

Differential Revision: https://secure.phabricator.com/D19687