Update flags for getting exception messages in docs (#18538)

`-lembind` is now preferred over `--bind`.

Add `-sEXPORT_EXCEPTION_HANDLING_HELPERS` to export the required symbols, which are now multiple and can vary based on exception handling backend.

site/source/docs/porting/Debugging.rst

@@ -170,7 +170,7 @@ returns the ``what`` function call result.
     emscripten::function("getExceptionMessage", &getExceptionMessage);
   };
 
-This requires using the linker flag ``--bind``.
+This requires using the linker flags ``-lembind -sEXPORT_EXCEPTION_HANDLING_HELPERS``.
 Once such a function has been created, exception handling code in javascript
 can call it when receiving an exception from WASM. Here the function is used
 in order to log the thrown exception.