add symbol export iterator for Mach-O

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on February 09, 2014 19:35:05_

I'm starting to split off pieces of the original issue #58 that are likely to be delayed beyond core support. The symbol export iterator is one, and it deserves its own issue now as it's holding up drltrace, which is disabled on Mac until this is implemented.

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=1360_