RFR: JDK-8306304: Fix xlc17 clang warnings in ppc and aix code

[JoKern65]

When using the new xlc17 compiler (based on a recent clang) to build OpenJDk on AIX , we run into various "warnings as errors".
Many of those are in the aix or ppc specific codebase and could be addressed by small adjustments.
A lot of those changes are in hotspot, some might be somewhere else in the OpenJDK C/C++ code.
With this PR we address only the platform dependent code changes.

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Commit messages:
 - JDK-8306304

Changes: https://git.openjdk.org/jdk/pull/13953/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13953&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8306304
  Stats: 36 lines in 9 files changed: 7 ins; 0 del; 29 mod
  Patch: https://git.openjdk.org/jdk/pull/13953.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13953/head:pull/13953

PR: https://git.openjdk.org/jdk/pull/13953