riscv: Add the clone3 wrapper

It follows the internal signature:

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

Checked on riscv64-linux-gnu-rv64imafdc-lp64d.

Reviewed-by: Palmer Dabbelt <palmer@rivosinc.com>