[foreign-abi] RFR: JDK-8244270: reorganize ABI-dependent layout constants (second attempt)

[Maurizio Cimadamore]

This is the same as https://git.openjdk.java.net/panama-foreign/pull/141 but without the jextract changes. Now that we
have reached consensus on the jextract changes, I will integrate this first, and merge the above PR later.

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Commit messages:
 - Refactor ABI constants

Changes: https://git.openjdk.java.net/panama-foreign/pull/147/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/147/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-JDK-8244270
  Stats: 1014 lines in 23 files changed: 234 ins; 660 del; 120 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/147.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/147/head:pull/147

PR: https://git.openjdk.java.net/panama-foreign/pull/147