RFR: JDK-8308288: Fix xlc17 clang warnings in shared 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".
Some of those are in shared 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.

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Commit messages:
 - white space
 - JDK-8308288

Changes: https://git.openjdk.org/jdk/pull/14146/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14146&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8308288
  Stats: 52 lines in 11 files changed: 46 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/14146.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14146/head:pull/14146

PR: https://git.openjdk.org/jdk/pull/14146