RFR: 8297451: ProcessHandleImpl should assert privilege when modifying reaper thread

[Ryan Ernst]

This commit guards thread modifications for the process reaper thread with doPrivileged.

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Commit messages:
 - 8297451: ProcessHandleImpl should assert privilege when modifying reaper thread

Changes: https://git.openjdk.org/jdk/pull/11309/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11309&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8297451
  Stats: 31 lines in 2 files changed: 19 ins; 1 del; 11 mod
  Patch: https://git.openjdk.org/jdk/pull/11309.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11309/head:pull/11309

PR: https://git.openjdk.org/jdk/pull/11309