riscv: enable sys_clone3 syscall for rv64

Enable the sys_clone3 syscall for RV64.  We simply include the generic
version.

Tested by running the program from

   https://lore.kernel.org/lkml/20190716130631.tohj4ub54md25dys@brauner.io/



and verifying that it completes successfully.

Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>
Acked-by: Christian Brauner <christian@brauner.io>
Cc: Christian Brauner <christian@brauner.io>