[foreign-abi] RFR: Revert "JDK-8242127: reorganize ABI-dependent layout constants"

[Maurizio Cimadamore]

This reverts commit f7e7b9d6e76e54b1b5d4ff97d2a034769652e132.

The latest reorganization of SystemABI constants did not work with the jextract code, for two reasons:

1) jextract internally converts C types into layouts, and then expects to emit source files which have the 'right'
constants (e.g. C_INT, if the layout is that of a C int)

2) for tests, for making sure that the layout of a given e.g. struct field matches the expected one

Of course (1) could be avoided by just re-creating the constant from scratch, but that would lead to footprint issues,
and would still not address (2).

So, I'm postponing for now - we'll reserve to make changes in this area in the future.

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Commit messages:
 - Revert "JDK-8242127: reorganize ABI-dependent layout constants"

Changes: https://git.openjdk.java.net/panama-foreign/pull/99/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/99/webrev.00
  Stats: 1227 lines in 24 files changed: 684 ins; 450 del; 93 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/99.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/99/head:pull/99

PR: https://git.openjdk.java.net/panama-foreign/pull/99