RFR: 8264123: add ThreadsList.is_valid() support

[Daniel D.Daugherty]

On Tue, 30 Mar 2021 22:54:55 GMT, David Holmes <dholmes at openjdk.org> wrote:

>> ThreadsLists need an is_valid() function and checks in various
>> places to help catch bugs where a ThreadsList is dangling.
>> 
>> Other minor changes:
>> - change raw `_threads_hazard_ptr` access to used `get_threads_hazard_ptr()`.
>> - `get_threads_hazard_ptr()` should be `const`.
>> - fix a couple of old typos.
>> 
>> Update: forgot to mention that this fix was tested with Mach5 Tier[1-8].
>
> Looks good.
> 
> Thanks,
> David

@dholmes-ora - Thanks for closing the loop on your initial review.
@robehn and/or @fisk - I could use a second pair of eyes on this one...

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PR: https://git.openjdk.java.net/jdk/pull/3255