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

[JoKern65]

On Fri, 12 May 2023 16:16:01 GMT, JoKern65 <duke at openjdk.org> wrote:

>> 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

I implemented all suggested changes. Thank you all for participating.

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PR Comment: https://git.openjdk.org/jdk/pull/13953#issuecomment-1551237290