[lworld] RFR: 8312417: [lworld] Replacing NULL with nullptr in x86 code

[Frederic Parain]

Replacing NULL with nullptr in x86 code.
Builds tested with Mach5 (tier1).

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Commit messages:
 - Replace NULL with nullptr in x86 code

Changes: https://git.openjdk.org/valhalla/pull/887/files
 Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=887&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8312417
  Stats: 76 lines in 8 files changed: 0 ins; 0 del; 76 mod
  Patch: https://git.openjdk.org/valhalla/pull/887.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/887/head:pull/887

PR: https://git.openjdk.org/valhalla/pull/887