Summary: public
I was using `dlsym` as a more elegant way of checking if the function was defined
to prevent crashes if running on an unsupported architecture, but Xcode might
strip the symbol, even with `.no_dead_strip`, if there's no references to the
compilation unit at all.
Replace it with an ugly `#if` that checks if it's any of the supported targets
and extern the function prototype.
Reviewed By: jspahrsummers
Differential Revision: D2581143
fb-gh-sync-id: b004ed351de97f96c04b4a6c914ce55cfcbbbcbb
Tadeu Zagallo