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>

arch/riscv/include/uapi/asm/unistd.h

@@ -18,6 +18,7 @@
 #ifdef __LP64__
 #define __ARCH_WANT_NEW_STAT
 #define __ARCH_WANT_SET_GET_RLIMIT
+#define __ARCH_WANT_SYS_CLONE3
 #endif /* __LP64__ */
 
 #include <asm-generic/unistd.h>