cmake: Factor out CPU architecture detection (#13883)

* cmake: Define function for querying architecture

The tests and results match exactly those of ggml/src/CMakeLists.txt

* Switch arch detection over to new function

ggml/src/CMakeLists.txt

@@ -109,6 +109,8 @@ if (MSVC)
 else ()
     set(CMAKE_GENERATOR_PLATFORM_LWR "")
 endif ()
+ggml_get_system_arch()
+message(STATUS "GGML_SYSTEM_ARCH: ${GGML_SYSTEM_ARCH}")
 
 if (NOT MSVC)
     if (GGML_STATIC)