RFR: 8359923: Const accessors for the Deferred class

[Joel Sikström]

Hello,

This RFE adds const accessors to the `Deferred` class. We plan on using this in a future patch in ZGC. Thanks!

Testing:
 * Currently running tier1-2
 * Works in an WIP ZGC patch

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Commit messages:
 - 8359923: Const accessors for the Deferred class

Changes: https://git.openjdk.org/jdk/pull/25874/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25874&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8359923
  Stats: 13 lines in 1 file changed: 13 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/25874.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25874/head:pull/25874

PR: https://git.openjdk.org/jdk/pull/25874