RFR: 7903933: Move sharable items from different generations to a common file [v6]

[Nizar Benalla]

> Please review this patch to move the `C_*` layouts and the static utility methods into separate classes: `LayoutUtils.java` and `FFMUtils.java`, respectively.
> 
> - The names could later be personalized through a JSON configuration.
> - We can use static imports if the `-t` option is no used and the files are generated into the default package, in that case we use the classname to call the static methods or use the `C_*` constants.
> 
> Some tests had to be modified slightly, either by adding new static imports or replacing classnames.

Nizar Benalla has updated the pull request incrementally with one additional commit since the last revision:

  make utility classes final with a private constructor

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Changes:
  - all: https://git.openjdk.org/jextract/pull/278/files
  - new: https://git.openjdk.org/jextract/pull/278/files/01c8205f..532ee1a6

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jextract&pr=278&range=05
 - incr: https://webrevs.openjdk.org/?repo=jextract&pr=278&range=04-05

  Stats: 24 lines in 3 files changed: 18 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jextract/pull/278.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/278/head:pull/278

PR: https://git.openjdk.org/jextract/pull/278