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

[David Holmes]

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

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Commit messages:
 - 8269652: Factor out the common code for creating system j.l.Thread objects

Changes: https://git.openjdk.java.net/jdk/pull/4663/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4663&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8269652
  Stats: 140 lines in 8 files changed: 43 ins; 87 del; 10 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4663.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4663/head:pull/4663

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