ggml : add logging for native build options/vars (whisper/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
```

ggml/CMakeLists.txt

@@ -100,6 +100,10 @@ else()
     set(INS_ENB ON)
 endif()
 
+message(DEBUG "GGML_NATIVE         : ${GGML_NATIVE}")
+message(DEBUG "GGML_NATIVE_DEFAULT : ${GGML_NATIVE_DEFAULT}")
+message(DEBUG "INS_ENB             : ${INS_ENB}")
+
 option(GGML_CPU_HBM          "ggml: use memkind for CPU HBM" OFF)
 option(GGML_CPU_AARCH64      "ggml: use runtime weight conversion of Q4_0 to Q4_X_X" ON)
 option(GGML_CPU_KLEIDIAI     "ggml: use KleidiAI optimized kernels if applicable" OFF)