Hack up partial ehabi descriptor parsing. It's incomplete, and how I'm hooking it up is almost certainly wrong.

Before this patch, exceptions are thrown, the language independent unwinding seems to work, but there doesn't seem to be any code path that can return the _URC_HANDLER_FOUND. Reading through the spec, it looked like we needed to walk the descriptors after the personality routine to find the matching catch blocks and then invoke the __cxa_type_match() escape hatch function to delegate type matching back up into the C++ layer.

This is an attempt to hack in support for type matching.  However, stepping through the code, none of the exception frames have any descriptors after them. Confused.