RFR: 7903932: Make private static classes final and have a private constructor

[Nizar Benalla]

Please review this patch to make static utility classes final with a private constructor, as they are not meant to be extended.

This is a small refractor of the generated code.

TIA

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Commit messages:
 - fix whitespace errors and add blank line
 - make private utility classes final with a private constructor

Changes: https://git.openjdk.org/jextract/pull/274/files
  Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=274&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903932
  Stats: 12 lines in 1 file changed: 9 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jextract/pull/274.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/274/head:pull/274

PR: https://git.openjdk.org/jextract/pull/274