[jdk17u-dev] RFR: 8269466: Factor out the common code for initializing and starting internal VM JavaThreads

[Martin Doerr]

Backport of [JDK-8269466](https://bugs.openjdk.org/browse/JDK-8269466).

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Commit messages:
 - Backport 4107dcf6ec3fb7fd6eb0e12cdc404a2dee9ccaa1

Changes: https://git.openjdk.org/jdk17u-dev/pull/1498/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=1498&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8269466
  Stats: 285 lines in 13 files changed: 65 ins; 139 del; 81 mod
  Patch: https://git.openjdk.org/jdk17u-dev/pull/1498.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev.git pull/1498/head:pull/1498

PR: https://git.openjdk.org/jdk17u-dev/pull/1498