RFR: 8337785: Fix simple -Wzero-as-null-pointer-constant warnings in x86 code

[Kim Barrett]

Please review this trivial change that replaces some uses of literal 0 as a
null pointer constant in x86 code to instead use nullptr.

Testing: mach5 tier1

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Commit messages:
 - fix simple x86

Changes: https://git.openjdk.org/jdk/pull/20453/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20453&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8337785
  Stats: 13 lines in 5 files changed: 0 ins; 0 del; 13 mod
  Patch: https://git.openjdk.org/jdk/pull/20453.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20453/head:pull/20453

PR: https://git.openjdk.org/jdk/pull/20453