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: 8f3220a806 ("arch: wire-up clone3() syscall")
Signed-off-by: Christian Brauner <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: David Howells <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Adrian Reber <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Florian Weimer <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]

arch/xtensa/include/asm/unistd.h

@@ -3,6 +3,7 @@
 #define _XTENSA_UNISTD_H
 
 #define __ARCH_WANT_SYS_CLONE
+#define __ARCH_WANT_SYS_CLONE3
 #include <uapi/asm/unistd.h>
 
 #define __ARCH_WANT_NEW_STAT