RFR: 8327994: Update code gen in CallGeneratorHelper

[Jorn Vernee]

Update the code gen code in CallGeneratorHelper to reflect the latest state of the libTest(Downcall/Upcall)(Stack).c and shared.h files.

- The previous code wanted users to pipe stdout into a file. But, since we have 5 files that need to be created, and the names of those files is important too, I've changed the script to write to those files directly instead.
- I moved the definition of `S_FFFF` to libVarArgs.c where it's used, as it's not actually generated by CallGeneratorHelper.
- GitHub doesn't render the changes to some of the files, but those files only contain either white space changes (some missing spaces after `,`), and copyright header updates.

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Commit messages:
 - add more run instructions
 - simplify
 - use export.h
 - factor out some shared code
 - fix test file generation
 - remove dead code from CallGeneratorHelper

Changes: https://git.openjdk.org/jdk/pull/18269/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18269&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8327994
  Stats: 36078 lines in 6 files changed: 62 ins; 40 del; 35976 mod
  Patch: https://git.openjdk.org/jdk/pull/18269.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18269/head:pull/18269

PR: https://git.openjdk.org/jdk/pull/18269