RFR: 7903270: Util classes should be final and have a private constructor [v6]

[Per Minborg]

> This PR makes some of the classes and fields `final`. Also, a general cleanup of the `RuntimeHelper` class is proposed.
> 
> There are some files that are "overhidden" (by attempting to "override"/hide static methods). This will be fixed in a separate PR and so these classes are not affected in this PR.

Per Minborg has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains seven commits:

 - Merge panama
 - Merge branch 'panama' into util-final
 - Add option to show generating class
 - Make Constants package private
 - Make constants package private
 - Remove debug output
 - Make some classes/fields final and add privat ctr

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Changes: https://git.openjdk.org/jextract/pull/65/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=65&range=05
  Stats: 120 lines in 6 files changed: 70 ins; 19 del; 31 mod
  Patch: https://git.openjdk.org/jextract/pull/65.diff
  Fetch: git fetch https://git.openjdk.org/jextract pull/65/head:pull/65

PR: https://git.openjdk.org/jextract/pull/65