RFR: 8264123: add ThreadsList.is_valid() support

[David Holmes]

On Mon, 29 Mar 2021 21:48:23 GMT, Daniel D. Daugherty <dcubed 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

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Marked as reviewed by dholmes (Reviewer).

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