RFR: 8193559: ugly DO_JAVA_THREADS macro should be replaced [v2]

[Daniel D.Daugherty]

On Sun, 4 Jul 2021 22:29:42 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

>> Daniel D. Daugherty has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   kbarrett CR - delete unused _list member.
>
> Changes requested by kbarrett (Reviewer).

I've updated the fix with @kimbarrett's rewrite that uses newer C++ features.
I've tested this version with Mach5 Tier[1-3].

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PR: https://git.openjdk.java.net/jdk/pull/4671