FFI: Add unwind definitions for lj_vm_ffi_call.

Adds exception interoperability for C/C++ functions called via FFI
from the interpreter.

doc/ext_ffi_semantics.html

@@ -1005,7 +1005,8 @@ Other missing features:
 <li>Callbacks from C&nbsp;code to Lua functions.</li>
 <li>Passing structs by value to vararg C&nbsp;functions.</li>
 <li><a href="extensions.html#exceptions">C++ exception interoperability</a>
-does not extend to C&nbsp;functions called via the FFI.</li>
+does not extend to C&nbsp;functions called via the FFI, if the call is
+compiled.</li>
 </ul>
 <br class="flush">
 </div>