Add XDEBUG support for OS X

by Timothy Savard for Mono Project

To provide equivalent support for emitting .dSYM files on OS X as for emitting .so files on platforms, thus allowing use of native OS X debugging tools for JIT debugging. This support should also be implemented such that it compartmentalizes the generation of .dSYM files as much as possible, minimizing impact on the rest of the runtime. In doing so, a framework for adding other alternate symbol file formats in the future could be laid out.