RFR: 8339120: Use more fine-granular gcc unused warnings [v5]

[Magnus Ihse Bursie]

> 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.

Magnus Ihse Bursie has updated the pull request incrementally with one additional commit since the last revision:

  Fix s390x build

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Changes:
  - all: https://git.openjdk.org/jdk/pull/20733/files
  - new: https://git.openjdk.org/jdk/pull/20733/files/69a6b308..f4bdce72

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20733&range=04
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20733&range=03-04

  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/20733.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20733/head:pull/20733

PR: https://git.openjdk.org/jdk/pull/20733