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

[JoKern65]

On Fri, 12 May 2023 12:01:43 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.

Explained change in a new comment
-32768 means same as ((-1)<<15)) , but the compiler doesn't like this anymore.

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

PR Comment: https://git.openjdk.org/jdk/pull/13953#issuecomment-1545976254