RISC-V: Include clone3() on rv32

As far as I can tell this should be enabled on rv32 as well, I'm not
sure why it's rv64-only.  checksyscalls is complaining about our lack of
clone3() on rv32.

Fixes: 56ac5e21 ("riscv: enable sys_clone3 syscall for rv64")
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>