Fix hhvm-dbg to actually include debugging symbols

The hhvm-dbg package right now is not very useful, as HHVM is built
without -g. Fixing this needed an upstream fix, however, as
RelWithDebInfo CMAKE target right now does not pass NDEBUG, resulting in
a (broken) debug build.

Change-Id: I23f587c0fb6d7db424d18c3341b1e0c5248fa2ba