RFR: JDK-8324682 Remove redefinition of NULL for XLC compiler [v6]

[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 updated the pull request incrementally with one additional commit since the last revision:

   update copyright

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Changes:
  - all: https://git.openjdk.org/jdk/pull/18064/files
  - new: https://git.openjdk.org/jdk/pull/18064/files/ec40c344..cc5f5bca

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18064&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18064&range=04-05

  Stats: 2 lines in 1 file changed: 0 ins; 1 del; 1 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