ggml : add logging for native build options/vars (#2935)

ggml : add logging for native build options/vars (#2935)

This commit adds debug level logging for the native build options and
variables to ggml/CMakeLists.txt.

The motivation for this is that it can be useful to see the effective
result of `GGML_NATIVE`, `GGML_NATIVE_DEFAULT`, and `INS_ENB` for a
cmake build. I've found myself adding similar logging a few times now,
so I thought it might be a good idea to add this.

Example output, specifying `-DCMAKE_MESSAGE_LOG_LEVEL=DEBUG` when
running cmake produces the following output:
```console
-- GGML_NATIVE         : OFF
-- GGML_NATIVE_DEFAULT : OFF
-- INS_ENB             : OFF
```