RFR: JDK-8324682 Remove redefinition of NULL for XLC compiler

[Suchismith Roy]

Remove redefinition of NULL for XLC compiler

JBS ISSUE : [JDK-8324682](https://bugs.openjdk.org/browse/JDK-8324682)

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Commit messages:
 - Redefine NULL word
 - Remove NULL definition

Changes: https://git.openjdk.org/jdk/pull/18064/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18064&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8324682
  Stats: 17 lines in 3 files changed: 0 ins; 14 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/18064.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18064/head:pull/18064

PR: https://git.openjdk.org/jdk/pull/18064