RFR: 8339120: Use more fine-granular gcc unused warnings

[Magnus Ihse Bursie]

On Tue, 27 Aug 2024 20:04:21 GMT, Magnus Ihse Bursie <ihse at openjdk.org> wrote:

> Currently, we issue -Wno-unused for all files in gcc, which is a rather big sledgehammer to get rid of some warnings that proliferate in a few areas of the build.
> 
> We should instead leave -Wunused turned on (as done by -Wall) and use a much more fine-grained approach to disabling specific warnings in specific files or libraries.

If at some point the diff seems larger than needed, it could be that I have reordered things where the alphabetic order had capsized.

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PR Comment: https://git.openjdk.org/jdk/pull/20733#issuecomment-2313681273