[S390] cpumask: Use accessors code.

Impact: use new API

Use the accessors rather than frobbing bits directly.  Most of this is
in arch code I haven't even compiled, but is straightforward.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Mike Travis <travis@sgi.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>