sparc64: allocate membarrier system call number

Now that the membarrier system call is allocated on sparc, 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>