RFR: 8308092: Replace NULL with nullptr in gc/x

[Stefan Karlsson]

Replace NULL with nullptr in gc/x. We've already done this work for Generational ZGC, but left it for the Singlegen ZGC code.

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Commit messages:
 - 8308092: Replace NULL with nullptr in gc/x

Changes: https://git.openjdk.org/jdk/pull/13984/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13984&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8308092
  Stats: 252 lines in 54 files changed: 0 ins; 0 del; 252 mod
  Patch: https://git.openjdk.org/jdk/pull/13984.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13984/head:pull/13984

PR: https://git.openjdk.org/jdk/pull/13984