libcxxabi contains the runtime support for C++.  But, as some folks have
realized, it is not complete.  It relies on some _Unwind_* functions to be
supplied by the OS. That means it cannot be ported to platforms that don’t
already have an unwinder.

Years ago Apple wrote its own unwinder for MacOSX and iOS.  To make libcxxabi
complete, Apple has decided the source code for its unwinder can be contributed
to the open source LLVM libcxxabi project, with a dual licensed under LLVM
and MIT license.

So, I’ve spent some time cleaning up the sources to make them conform with
LLVM style and to conditionalize the sources in a way that should make it
easier to port to other platforms.  The sources are in a separate "Unwind"
directory under "src" in libcxxabi.

Background:
Most architectures now use "zero cost" exceptions for C++.  The zero cost means
there are no extra instructions executed if no exceptions are thrown.  But if
an exception is thrown, the runtime must consult side tables and figure out how
to restore registers and "unwind" from the current stack frame to the catch
clause.  That ability to modify the stack frames and cause the thread to resume
in a catch clause with all registers restored properly is the main purpose
of the unwinder.

This unwinder has two levels of API.  The high level APIs are the _Unwind_*
functions which the cxa_* exception functions in libcxxabi require.  The low
level APIs are the unw_* functions which are an interface defined by the the
old HP libunwind project (which shares no code with this unwinder).