arm: allocate membarrier system call number

Now that the membarrier system call is allocated on ARM, allocate its
number in our architecture header if the system headers don't allocate
it. This allows using the membarrier system call as soon as implemented
in the kernel, even if the distribution has old kernel headers.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>