Unwind error handler library

This library provides an error handler that unwinds using the APIs in unwind.h.

This error handler can work even in cases of stack overflow.

Note: Unlike most libraries, this is not built as a shared library. Error handlers must be part of the compartment that invokes them. As a result, this must be added as a dependency of each compartment that wishes to use it, rather than as a dependency of the firmware target.