RFR: 8269652: Factor out the common code for creating system j.l.Thread objects

[David Holmes]

On Fri, 2 Jul 2021 07:03:50 GMT, David Holmes <dholmes at openjdk.org> wrote:

> Please review this simple refactoring to share the common code used to create j.l.Thread instances for the internal VM JavaThreads. (Also fix a missing space from my previous change in thread.cpp.)
> 
> It is all very straight-forward.
> 
> Compiler folk I can go one step further and remove CompileBroker::create_thread_oop if desired.
> 
> Testing: tiers 1-3, GHA
> 
> Thanks,
> David

Thanks for all the reviews!

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PR: https://git.openjdk.java.net/jdk/pull/4663