RFR: 8308286 Fix clang warnings in linux code

[Daniel Jeliński]

On Wed, 17 May 2023 12:28:47 GMT, Artem Semenov <asemenov at openjdk.org> wrote:

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

According to our docs, [clang is a supported compiler for Linux](https://github.com/openjdk/jdk/blob/master/doc/building.md#native-compiler-toolchain-requirements).

Here's an example how warning exclusion is implemented today:
https://github.com/openjdk/jdk/blob/master/make/modules/java.desktop/lib/Awt2dLibraries.gmk#L827

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PR Comment: https://git.openjdk.org/jdk/pull/14033#issuecomment-1563956337