parisc: allocate sys_membarrier system call number

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Tested-by: Helge Deller <deller@gmx.de>
CC: Andrew Morton <akpm@linux-foundation.org>
CC: linux-api@vger.kernel.org
CC: "James E.J. Bottomley" <jejb@parisc-linux.org>
CC: linux-parisc@vger.kernel.org
Signed-off-by: Helge Deller <deller@gmx.de>