hppa: allocate membarrier system call number

Now that the membarrier system call is allocated on hppa (parisc),
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>