cmake : avoid duplicate -march=native when already set in CFLAGS

If -march= is already specified via CMAKE_C_FLAGS/CMAKE_CXX_FLAGS or
ARCH_FLAGS, do not append -march=native again. This prevents compiler
warnings about redundant flags and allows users to override the march
target by setting it in their build environment.

Fixes duplicate -march flag in ggml-cpu and ggml-amx backends.

ggml/src/ggml-cpu/CMakeLists.txt

@@ -311,7 +311,10 @@ function(ggml_add_cpu_backend_variant_impl tag_name)
             endif()
         else ()
             if (GGML_NATIVE)
-                list(APPEND ARCH_FLAGS -march=native)
+               string(REGEX MATCH "-march=[^ ]+" MARCH_MATCH "${CMAKE_C_FLAGS} ${CMAKE_CXX_FLAGS}")
+               if(NOT MARCH_MATCH)
+                  list(APPEND ARCH_FLAGS -march=native)
+               endif()
             else ()
                 if (GGML_SSE42)
                     list(APPEND ARCH_FLAGS -msse4.2)