RFR: JDK-8324682 Remove redefinition of NULL for XLC compiler [v3]

[Suchismith Roy]

> Remove redefinition of NULL for XLC compiler
> In globalDefinitions_xlc.hpp there is a redefinition of NULL to work around an issue with one of the AIX headers (<dirent.h>). 
> Once all uses of NULL in HotSpot have been replaced with nullptr, there is no longer any need for this, and it can be removed.
> 
> 
> JBS ISSUE : [JDK-8324682](https://bugs.openjdk.org/browse/JDK-8324682)

Suchismith Roy has refreshed the contents of this pull request, and previous commits have been removed. The incremental views will show differences compared to the previous content of the PR. The pull request contains one new commit since the last revision:

  remove AIX check for NULL

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Changes:
  - all: https://git.openjdk.org/jdk/pull/18064/files
  - new: https://git.openjdk.org/jdk/pull/18064/files/eb8faf93..21fd7a2c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18064&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18064&range=01-02

  Stats: 226 lines in 1 file changed: 226 ins; 0 del; 0 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