lib/on-cpus: Add barrier after func call

It's reasonable for users of on_cpu() and on_cpumask() to assume
they can read data that 'func' has written when the call completes.
Ensure the caller doesn't observe a completion (target cpus are again
idle) without also being able to observe any writes which were made
by func(). Do so by adding barriers to implement the following

 target-cpu                                   calling-cpu
 ----------                                   -----------
 func() /* store something */                 /* wait for target to be idle */
 smp_wmb();                                   smp_rmb();
 set_cpu_idle(smp_processor_id(), true);      /* load what func() stored */

Suggested-by: Alexandru Elisei <alexandru.elisei@arm.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Andrew Jones <andrew.jones@linux.dev>