Override default tuning which is for the build system

--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -175,18 +175,34 @@
     string(SUBSTRING "${_GCC_OUTPUT}" ${POS} -1 _GCC_OUTPUT)
     string(REGEX REPLACE "${ARCH_FLAG}=[ \t]*([^ \n]*)[ \n].*" "\\1" GNUCC_ARCH "${_GCC_OUTPUT}")
 
-    # test the parsed flag
-    set (EXEC_ARGS ${CC_ARG1} -E - -mtune=${GNUCC_ARCH})
-    execute_process(COMMAND ${CMAKE_C_COMPILER} ${EXEC_ARGS}
-        OUTPUT_QUIET ERROR_QUIET
-        INPUT_FILE /dev/null
-        RESULT_VARIABLE GNUCC_TUNE_TEST)
-    if (NOT GNUCC_TUNE_TEST EQUAL 0)
-        message(WARNING "Something went wrong determining gcc tune: -mtune=${GNUCC_ARCH} not valid, falling back to -mtune=native")
-        set(TUNE_FLAG native)
+    if (CMAKE_SYSTEM_PROCESSOR MATCHES "arch64")
+          set(GNUCC_ARCH armv8)
+          set(TUNE_FLAG generic)
+          message(STATUS "gcc will tune for ${GNUCC_ARCH}, ${TUNE_FLAG}")
+          set(ARCH_C_FLAGS -march=armv8-a)
+          set(ARCH_CXX_FLAGS -march=armv8-a)
+          message(WARNING "Using armv8-a for arm64")
+    elseif(CMAKE_SYSTEM_PROCESSOR MATCHES "x86_64" OR CMAKE_SYSTEM_PROCESSOR MATCHES "i686")
+          set(GNUCC_ARCH core2)
+          set(TUNE_FLAG generic)
+          message(STATUS "gcc will tune for ${GNUCC_ARCH}, ${TUNE_FLAG}")
+          set(ARCH_C_FLAGS -march=core2)
+          set(ARCH_CXX_FLAGS -march=core2)
+          message(WARNING "Using core2 for X86/X86_64")
     else()
-        set(TUNE_FLAG ${GNUCC_ARCH})
-        message(STATUS "gcc will tune for ${GNUCC_ARCH}, ${TUNE_FLAG}")
+      # test the parsed flag
+      set (EXEC_ARGS ${CC_ARG1} -E - -mtune=${GNUCC_ARCH})
+      execute_process(COMMAND ${CMAKE_C_COMPILER} ${EXEC_ARGS}
+          OUTPUT_QUIET ERROR_QUIET
+          INPUT_FILE /dev/null
+          RESULT_VARIABLE GNUCC_TUNE_TEST)
+      if (NOT GNUCC_TUNE_TEST EQUAL 0)
+          message(WARNING "Something went wrong determining gcc tune: -mtune=${GNUCC_ARCH} not valid, falling back to -mtune=native")
+            set(TUNE_FLAG native)
+      else()
+          set(TUNE_FLAG ${GNUCC_ARCH})
+          message(STATUS "gcc will tune for ${GNUCC_ARCH}, ${TUNE_FLAG}")
+      endif()
     endif()
 elseif (CMAKE_COMPILER_IS_CLANG AND NOT CROSS_COMPILE)
     if (ARCH_IA32 OR ARCH_X86_64)