Build PHP extension with HHVM (non voting)

Build FastStringSearch, Wikidiff2 and LuaSandbox with HHVM on Trusty
instances.

Triggers non voting jobs::
    php-FastStringSearch-hhvm-build
    php-luasandbox-hhvm-build
    php-wikidiff2-hhvm-build

Bug: 63120
Change-Id: Ic7ca7b371e2542b52a8aecc65be2917928dfad6d