RFR: 8308286 Fix clang warnings in linux code

[Artem Semenov]

When using the clang compiler to build OpenJDk on Linux, we encounter various "warnings as errors".
They can be fixed with small changes.

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Commit messages:
 - 8308286 Fix clang warnings in linux code

Changes: https://git.openjdk.org/jdk/pull/14033/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14033&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8308286
  Stats: 134 lines in 12 files changed: 126 ins; 0 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/14033.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14033/head:pull/14033

PR: https://git.openjdk.org/jdk/pull/14033