Fix regression on OS X introduced by b53a42309061876adf275c82ff87df3981fe1bf2

Summary: OS X has some issues with __attribute__((weak)), see http://glandium.org/blog/?p=2764 for more information.
Closes https://github.com/facebook/hhvm/pull/3398

Reviewed By: @ptarjan

Differential Revision: D1484825Pulled By: @sgolemon