Support reading random bytes on Windows

Summary:
We use this quite extensively, e.g. in PhutilOpaqueEnvelope.
Filesystem is not the best location for this but I'm not in a mood of renaming it everywhere.
It doesn't work by default but better than nothing.

Test Plan:

phabricator\src\infrastructure\storage\ $ arc unit __tests__

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3860