Move generic start code and memset into arch/generic.h

buildtest.c

@@ -60,13 +60,3 @@ int main() {
 
   return 69;
 }
-
-void __libc_start_main() {exit(main());}
-
-void _start() {
-  __libc_start_main();
-}
-
-void __start() {
-  _start();
-}