libunwind should install libunwind_ext.h to simplify the code of JIT Engines

[Naville]

For example, orc/macho_platform.cpp does a manual definition of compact unwind dynamic registration APIs.

Maybe CC @lhames

We are implementing OrcJIT based JIT Implementation on Darwin and hit this exact same issue (MachOPlatform isn't an option for us, for reasons)

Defining a local implementation doesn't seem to be a good idea due to:

• Not sure at which point is libSystem's libunwind is updated to include this.
• Not aware if the function prototype will change (and thus require manual updating in the future)
• Want clear compile-time error when compiling with an old macOS SDK