RFR: JDK-8306304: Fix xlc17 clang warnings in ppc and aix code [v2]

[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.

JoKern65 has updated the pull request incrementally with one additional commit since the last revision:

  cosmetic changes

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Changes:
  - all: https://git.openjdk.org/jdk/pull/13953/files
  - new: https://git.openjdk.org/jdk/pull/13953/files/ad3be1bd..d7e2d4f9

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13953&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13953&range=00-01

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