RFR: 7903270: Util classes should be final and have a private constructor [v3]

[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:

  Make Constants package private

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

Changes:
  - all: https://git.openjdk.org/jextract/pull/65/files
  - new: https://git.openjdk.org/jextract/pull/65/files/2a7fe21f..5a070770

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jextract&pr=65&range=02
 - incr: https://webrevs.openjdk.org/?repo=jextract&pr=65&range=01-02

  Stats: 5 lines in 1 file changed: 5 ins; 0 del; 0 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