Unless cpu0 code explicitly calls do_idle(), then it never will,
and thus any on_cpu(0, ...) calls will hang. Let's assume that
if cpu0 hasn't already called do_idle() that the on_cpu(0, ...)
call from a secondary CPU was a programming error and assert.
However, it's possible the developer intends cpu0 to call do_idle
later, so give the developer a chance to disable the assert too.

Also, while cpu0 is special, it's not that special. It shouldn't
be the only CPU that can call on_cpus().

Finally, let's not mess with cpu0's idle state in on_cpus(), as
this could potentially confuse another CPU that's checking it.

Signed-off-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>