Implement the magic libunwind library

[robdockins]

CF http://refspecs.linuxfoundation.org/abi-eh-1.22.html

Implementing the _Unwind level of the APIs discussed in the above link, will, I think, allow us to directly reuse the actual implementation of the libcxxabi from LLVM to deal accurately with C++ exceptions. AFAICT, all the magic stack introspection steps actually happen inside libunwind, and is therefore an appropriate abstraction layer for override implementation inside Crucible.

I haven't completely convinced myself that the operations so described are actually reasonable to bolt onto crucible, but it doesn't seem impossible.

[kquick]

Will also be useful for handling JVM exceptions.

[kquick]

May be related to setjmp/longjmp.