RFR: 8339120: Use more fine-granular gcc unused warnings [v2]

[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:

  Use -Wunused-const-variable=1 instead

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/20733/files
  - new: https://git.openjdk.org/jdk/pull/20733/files/7d4f1380..27cd3b06

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20733&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20733&range=00-01

  Stats: 48 lines in 5 files changed: 8 ins; 1 del; 39 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