RFR: 8271624: Avoid unnecessary ThreadGroup.checkAccess calls when creating Threads

[Claes Redestad]

Trivially avoid some redundant calls when creating threads. (ThreadGroup.checkAccess is final, and all it does is call security.checkAccess(g) if there's a SM installed)

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Commit messages:
 - Merge branch 'master' into thread_access
 - Avoid calling ThreadGroup.checkAccess when no SecurityManager is installed

Changes: https://git.openjdk.java.net/jdk/pull/4959/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4959&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8271624
  Stats: 8 lines in 1 file changed: 4 ins; 4 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4959.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4959/head:pull/4959

PR: https://git.openjdk.java.net/jdk/pull/4959