RFR: JDK-8306304: Fix xlc17 clang warnings in ppc and aix code [v3]

[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:

  followed the suggested changes

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/13953/files
  - new: https://git.openjdk.org/jdk/pull/13953/files/d7e2d4f9..c3bd97b4

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13953&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13953&range=01-02

  Stats: 12 lines in 2 files changed: 4 ins; 3 del; 5 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