add an option to mark dstacks as executable

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on September 25, 2014 10:08:29_

Tool code that uses gcc nested functions needs executable stacks. We may want to provide an option to make dstacks executable (off by default). One user porting a tool from Pin hit this: the code used twalk and worked on Pin (which must have executable stacks).

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=1545_