ARC: Add the clone3 wrapper

Use the clone3 wrapper on ARC. It doesn't care about stack alignment.
All callers should provide an aligned stack.
It follows the internal signature:

extern int clone3 (struct clone_args *__cl_args, size_t __size,
 int (*__func) (void *__arg), void *__arg);

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>