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

[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.

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Commit messages:
 - Fix debug builds
 - Try fixing static builds on zero
 - Fix zero
 - Fix libfallbackLinker
 - Also update native tests
 - 8339120: Use more fine-granular gcc unused warnings

Changes: https://git.openjdk.org/jdk/pull/20733/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20733&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8339120
  Stats: 50 lines in 17 files changed: 28 ins; 0 del; 22 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