un-alias cpumask_any() from cpumask_first()

un-alias cpumask_any() from cpumask_first()

In order to achieve more symmetric distribution of certain things,
cpumask_any() shouldn't always pick the first CPU (which frequently
will end up being CPU0). To facilitate that, introduce a library-like
function to obtain random numbers.

The per-architecture function is supposed to return zero if no valid
random number can be obtained (implying that if occasionally zero got
produced as random number, it wouldn't be considered such).

As fallback this uses the trivial algorithm from the C standard,
extended to produce "unsigned int" results.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>
Reviewed-by: George Dunlap <george.dunlap@eu.citrix.com>