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

[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.

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Commit messages:
 - move sharable items to a common class

Changes: https://git.openjdk.org/jextract/pull/278/files
  Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=278&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903933
  Stats: 411 lines in 21 files changed: 233 ins; 106 del; 72 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