arch: handle arches who do not yet define clone3

This cleanly handles arches who do not yet define clone3.

clone3() was initially placed under __ARCH_WANT_SYS_CLONE under the
assumption that this would cleanly handle all architectures. It does
not.
Architectures such as nios2 or h8300 simply take the asm-generic syscall
definitions and generate their syscall table from it. Since they don't
define __ARCH_WANT_SYS_CLONE the build would fail complaining about
sys_clone3 missing. The reason this doesn't happen for legacy clone is
that nios2 and h8300 provide assembly stubs for sys_clone. This seems to
be done for architectural reasons.

The build failures for nios2 and h8300 were caught int -next luckily.
The solution is to define __ARCH_WANT_SYS_CLONE3 that architectures can
add. Additionally, we need a cond_syscall(clone3) for architectures such
as nios2 or h8300 that generate their syscall table in the way I
explained above.

Fixes: 8f3220a80654 ("arch: wire-up clone3() syscall")
Signed-off-by: Christian Brauner <[e-mail]>
Cc: Arnd Bergmann <[e-mail]>
Cc: Kees Cook <[e-mail]>
Cc: David Howells <[e-mail]>
Cc: Andrew Morton <[e-mail]>
Cc: Oleg Nesterov <[e-mail]>
Cc: Adrian Reber <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Cc: Al Viro <[e-mail]>
Cc: Florian Weimer <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]

arch/arm/include/asm/unistd.h

···
 #define __ARCH_WANT_SYS_FORK
 #define __ARCH_WANT_SYS_VFORK
 #define __ARCH_WANT_SYS_CLONE
+#define __ARCH_WANT_SYS_CLONE3
 
 /*
  * Unimplemented (or alternatively implemented) syscalls

arch/arm64/include/asm/unistd.h

···
 #endif
 
 #define __ARCH_WANT_SYS_CLONE
+#define __ARCH_WANT_SYS_CLONE3
 
 #ifndef __COMPAT_SYSCALL_NR
 #include <uapi/asm/unistd.h>

arch/x86/include/asm/unistd.h

···
 # define __ARCH_WANT_SYS_FORK
 # define __ARCH_WANT_SYS_VFORK
 # define __ARCH_WANT_SYS_CLONE
+# define __ARCH_WANT_SYS_CLONE3
 
 #endif /* _ASM_X86_UNISTD_H */

arch/xtensa/include/asm/unistd.h

···
 #define _XTENSA_UNISTD_H
 
 #define __ARCH_WANT_SYS_CLONE
+#define __ARCH_WANT_SYS_CLONE3
 #include <uapi/asm/unistd.h>
 
 #define __ARCH_WANT_NEW_STAT

kernel/fork.c

···
 
 	return _do_fork(&args);
 }
+#endif
 
+#ifdef __ARCH_WANT_SYS_CLONE3
 noinline static int copy_clone_args_from_user(struct kernel_clone_args *kargs,
 					      struct clone_args __user *uargs,
 					      size_t size)

kernel/sys_ni.c

···
 /* kernel/exit.c */
 
 /* kernel/fork.c */
+/* __ARCH_WANT_SYS_CLONE3 */
+COND_SYSCALL(clone3);
 
 /* kernel/futex.c */
 COND_SYSCALL(futex);