cpumask: Use accessors code in core

Impact: use new API

cpu_*_map are going away in favour of cpu_*_mask, but const pointers.
So we have accessors where we really do want to frob them.  Archs
will also need the (trivial) conversion before we can finally remove
cpu_*_map.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Mike Travis <travis@sgi.com>