RFR: 8305880: Loom: Avoid putting stale object pointers in oops

[Stefan Karlsson]

Generational ZGC has extra verification code for oops, which trigger asserts when it finds stale oops. We have cleaned away some usages of stale oops in the upstream repository (openjdk/jdk), but there are still a couple left in the Loom code. I propose that we rewrite the code, to pave the way for Generational ZGC.

I've tested this by running these patches on top of openjdk/fibers + ZGC. I've also tested this with Skynet + Generational ZGC, where these issues where first found.

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Commit messages:
 - Review Ron
 - Introduce derived_base type
 - Don't create stale oops in DerivedPointersSupport::relativize
 - Don't create stale oops in ~SafepointOp

Changes: https://git.openjdk.org/jdk/pull/13439/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13439&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8305880
  Stats: 48 lines in 10 files changed: 9 ins; 2 del; 37 mod
  Patch: https://git.openjdk.org/jdk/pull/13439.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13439/head:pull/13439

PR: https://git.openjdk.org/jdk/pull/13439