RFR: 7903270: Util classes should be final and have a private constructor [v4]

[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 incrementally with one additional commit since the last revision:

  Add option to show generating class

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Changes:
  - all: https://git.openjdk.org/jextract/pull/65/files
  - new: https://git.openjdk.org/jextract/pull/65/files/5a070770..29ce89c2

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jextract&pr=65&range=03
 - incr: https://webrevs.openjdk.org/?repo=jextract&pr=65&range=02-03

  Stats: 30 lines in 2 files changed: 21 ins; 7 del; 2 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