dbghelp search doesn't find exported symbols when have pdb

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on September 30, 2011 15:22:13_

today drmem looks for both b/c I've seen cases where dbghelp won't find
export symbols. should investigate further wrt different dbghelp versions,
whether pdb is available or not, etc.

should we change drsym_windows to do an export lookup whenever dbghelp
fails to find something? what about enum and search though?

should look again for a dbghelp flag or sthg that might control finding exports

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=568_