[lilliput-jdk17u:lilliput] RFR: 8310042: [Lilliput/JDK17] ZGC fixes and cleanups

[Roman Kennke]

In ZGC, we don't need to change zLiveMap.inline.hpp and can revert to upstream state. In zBarrierSetC2.cpp we need to relax an assert.

Testing:
- [x] hotspot_gc
- [x] tier -XX:+UseZGC

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Commit messages:
 - 8310042: [Lilliput/JDK17] ZGC fixes and cleanups

Changes: https://git.openjdk.org/lilliput-jdk17u/pull/31/files
 Webrev: https://webrevs.openjdk.org/?repo=lilliput-jdk17u&pr=31&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8310042
  Stats: 6 lines in 2 files changed: 4 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/lilliput-jdk17u/pull/31.diff
  Fetch: git fetch https://git.openjdk.org/lilliput-jdk17u.git pull/31/head:pull/31

PR: https://git.openjdk.org/lilliput-jdk17u/pull/31