From: Paul Jackson <pj@sgi.com>

Push the cpumask implementation from linux/cpumask.h into asm/cpumask.h, so
that ia64 can do special things without breaking sparc64.

1) Each arch has its own include/asm-<arch>/cpumask.h file

2) That arch-specific header file can include <asm-generic/cpumask.h>,
   if it wants to make use of the generic cpumask implementation.

3) Using code should continue to include linux/cpumask.h, which
   in turn includes asm/cpumask.h.  Some common implementation
   independent cpumask related items, such as the cpu_online_map,
   are declared directly in linux/cpumask.h.