RFR: 8338123: Linker crash when building a downcall handle with many arguments in x64

[Jorn Vernee]

- Adjust downcall stub sizes based on latest version. (per method described in https://github.com/openjdk/jdk/pull/12908)
- Beef up test for large stubs to also cover this particular case.

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Commit messages:
 - use junit
 - make test more rebust
 - add test
 - adjust downcall stub sizes

Changes: https://git.openjdk.org/jdk/pull/20842/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20842&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338123
  Stats: 31 lines in 2 files changed: 20 ins; 0 del; 11 mod
  Patch: https://git.openjdk.org/jdk/pull/20842.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20842/head:pull/20842

PR: https://git.openjdk.org/jdk/pull/20842