RFR: 8302067: [AIX] AIX build error on os_aix_ppc.cpp

[Varada M]

AIX build error on os_aix_ppc.cpp due to comparison between intptr_t with nullptr_t  can be resolved by type casting . The obtained error:

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Commit messages:
 - Aix build error fix

Changes: https://git.openjdk.org/jdk/pull/12482/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12482&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8302067
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/12482.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12482/head:pull/12482

PR: https://git.openjdk.org/jdk/pull/12482