RFR: 8341722: Fix some warnings as errors when building on Linux with toolchain clang [v2]

[Matthias Baesken]

> There are a few warnings as errors occurring when building on Linux with clang (clang15). Mostly these are some kind of 'unused' warnings.
> Might be related to https://bugs.openjdk.org/browse/JDK-8339156 .

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

  adjust gcc warning settings; bring back rslt

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/21407/files
  - new: https://git.openjdk.org/jdk/pull/21407/files/ee3546b1..705bd07b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21407&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21407&range=00-01

  Stats: 3 lines in 2 files changed: 1 ins; 2 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/21407.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21407/head:pull/21407

PR: https://git.openjdk.org/jdk/pull/21407