Inject 'wgPoweredByHHVM' JS config var if powered by HHVM

To be used by user scripts, etc. to mark HHVM-generated pages in some
user-visible way.

Change-Id: Ib44fe08981db8231ebecb5f14db3e01f2556f9b6
(cherry picked from commit 973c7340e75f69b59abfb730e989272eb8e22b75)