arm64: Move __ARCH_WANT_SYS_CLONE3 definition to uapi headers

Previously this was only defined in the internal headers which
resulted in __NR_clone3 not being defined in the user headers.

Signed-off-by: Amanieu d'Antras <amanieu@gmail.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: <stable@vger.kernel.org> # 5.3.x
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20200102172413.654385-2-amanieu@gmail.com


Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>