[foreign-abi] Integrated: Fix implicit default constructor linting warnings

[Jorn Vernee]

Hi,

This PR fixes some newly added linter warnings about implicit private constructor generation:

- Add private constructor to CSupport
- Make NativeScope and interface

wrt NativeScope; it had no fields, so it could be made an interface. I could also have added a `protected` empty
constructor, but making it an interface seemed cleaner (WDYT?)

Thanks,
Jorn

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Commit messages:
 - Fix implicit default constructor linting warnings:

Changes: https://git.openjdk.java.net/panama-foreign/pull/289/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/289/webrev.00
  Stats: 32 lines in 3 files changed: 6 ins; 0 del; 26 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/289.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/289/head:pull/289

PR: https://git.openjdk.java.net/panama-foreign/pull/289