generic_stop_cpus: pull timeout logic from under DIAGNOSTIC
generic_stop_cpus: pull timeout logic from under DIAGNOSTIC

... and also increase the timeout.
It's better to try to proceed somehow despite stuck CPUs than to hang
indefinitely.  Especially so during shutdown and when entering kdb or panic.

Timeout value is still an aribitrary value.
Timeout diagnostic is just a printf; the work on something more
debuggable is planned by attilio.  Need to be careful here as
stop_cpus_hard is called very early while enetering kdb and soon(-ish)
it may become called very early when entering panic.

Reviewed by:	attilio
MFC after:	2 months

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