sched/isolation: Add cpu_is_isolated() API

Provide this new API to check if a CPU has been isolated either through
isolcpus= or nohz_full= kernel parameter.

It aims at avoiding kernel load deemed to be safely spared on CPUs
running sensitive workload that can't bear any disturbance, such as
pcp cache draining.

Suggested-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>