aarch64: allocate membarrier system call number

aarch64: allocate membarrier system call number

Now that the membarrier system call is allocated on aarch64, 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.

Do so by creating headers specifically for aarch64, which rely on the
gcc atomic and memory barrier builtins.

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