openrisc: Enable the clone3 syscall

Enable the clone3 syscall for OpenRISC.  We use the generic version.

This was tested with the clone3 test from selftests.  Note, for all
tests to pass it required enabling CONFIG_NAMESPACES which is not
enabled in the default OpenRISC kernel config.

Signed-off-by: Stafford Horne <shorne@gmail.com>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>