Also, since its always present, remove the special case
code for adding systemlib to a RepoAuthoritative repo,
and clean up all the magic variables for finding systemlib.
If HHVM_SYSTEMLIB is set, and the file exists, it will be
used as systemlib, otherwise, the embedded one will be used.
This change is mostly for FB internal organizational reasons.
Building is not effected beyond the fact that the target now
lands in hphp/hhvm/hhvm rather than src/hhvm/hhvm.