sgi-xpc: clean up numerous globals

Introduce xpc_arch_ops and eliminate numerous individual global definitions.

Signed-off-by: Robin Holt <holt@sgi.com>
Cc: Dean Nelson <dcn@sgi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>