[foreign-jextract] RFR: 8277830: jextract generated code should not depend on Java layouts

[Maurizio Cimadamore]

This patch tweaks jextract generation, so that a new constant class is emitted, namely `Constant$root`, which contains the definition of all basic C primitive types. These are declared with the correct alignment constraints, depending on platforms.

Every other layout created by jextract will depend on these shared layouts constants, so that jextract code doesn't depend on alignment choices in `ValueLayout` constants.

In the future, we should probably generalize this approach, so that e.g. layouts are emitted only once, and then referred to afterwards - not just for primitive types, but also for everything else (struct, typedef, etc.). This would minimize the amount of layouts we store in the constant classes.

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Commit messages:
 - Declare primitive layout constants with sharp type, to avoid downstream casts
 - Cleanup code
 - Merge branch 'foreign-jextract' into primitive_layouts_jextract
 - Use C layouts in all constant classes.

Changes: https://git.openjdk.java.net/panama-foreign/pull/620/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=620&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8277830
  Stats: 148 lines in 8 files changed: 90 ins; 37 del; 21 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/620.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/620/head:pull/620

PR: https://git.openjdk.java.net/panama-foreign/pull/620