ARROW-12130: [C++] Don't enable Neon if -DARROW_SIMD_LEVEL=NONE

Closes apache#9830 from cyb70289/neon-simd-none

Authored-by: Yibo Cai <yibo.cai@arm.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>

cpp/cmake_modules/SetupCxxFlags.cmake

@@ -451,7 +451,9 @@ if(ARROW_CPU_FLAG STREQUAL "armv8")
   endif()
   set(CXX_COMMON_FLAGS "${CXX_COMMON_FLAGS} ${ARROW_ARMV8_ARCH_FLAG}")
 
-  add_definitions(-DARROW_HAVE_NEON)
+  if(NOT ARROW_SIMD_LEVEL STREQUAL "NONE")
+    add_definitions(-DARROW_HAVE_NEON)
+  endif()
 
   if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU"
      AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS "5.4")