disable all features for mac arm cpu-ref backend [no ci]

This commit disables all the cpu features for the mac arm cpu-ref
backend.
Before this commit the features reported when running a cpu variant
test were the following:
```console
CPU-ref features:
    NEON = 1
    ARM_FMA = 1
    FP16_VA = 1
    DOTPROD = 1
```
After this commit the features are:
```console
CPU-ref features:
  (no features reported)
```

Author: danbev

ggml/src/CMakeLists.txt

@@ -398,7 +398,20 @@ if (GGML_CPU_REF_BACKEND)
     set(GGML_OPENMP OFF)
     set(GGML_CPU_KLEIDIAI OFF)
     set(GGML_CPU_REPACK OFF)
+    set(GGML_ACCELERATE OFF)
+
     ggml_add_cpu_backend_variant(ref)
+
+    if (CMAKE_SYSTEM_PROCESSOR MATCHES "arm|aarch64|ARM|AARCH64")
+        target_compile_options(ggml-cpu-ref PRIVATE
+              -U__ARM_NEON
+              -U__ARM_FEATURE_FMA
+              -U__ARM_FEATURE_FP16_VECTOR_ARITHMETIC
+              -U__ARM_FEATURE_DOTPROD
+              -U__ARM_FEATURE_MATMUL_INT8
+              -U__ARM_FEATURE_SVE
+          )
+    endif()
     target_compile_definitions(ggml PRIVATE GGML_USE_CPU_REF)
 endif()