RFR: JDK-8308288: Fix xlc17 clang warnings in shared 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".
> 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.

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

  forgotton _

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Changes:
  - all: https://git.openjdk.org/jdk/pull/14146/files
  - new: https://git.openjdk.org/jdk/pull/14146/files/2699fdce..da38fb2d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=14146&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14146&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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