Per @mwilliams' suggestion, this is the first stage in a staggered approach to replacing int64 with int64_t. More precisely I inserted "typedef ::int64_t int64;" in util/base.h and dealt with the consequences.
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.
aalexandre