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

[Maurizio Cimadamore]

This is another attempt to reorganize ABI-dependent layout constants. The constant reorg is identical to the one
proposed in:

 https://git.openjdk.java.net/panama-foreign/pull/97

But this time I've also integrated the changes into the jextract branch (as illustrated in this PR).

The main realization which led to this work was that we don't need the ABI type enum: we already have the jextract
primitive type enum. So, we could just define constants for the common ABI types which are useful for ABI
classification in SystemABI - and then we could start from here in jextract, and augment these layouts by attaching an
extra attribute with jextract's Primitve/Kind enum.

I've simplified the API a bit by moving the layout inside the enum itself (so that the layout can be accessed directly
from the enum constant) - and also simplified the primitive type factory.

Everything worked out as expected - note how much duplicated layout creation we managed to get rid of.

One problem I had was that the loader we use inside jextract test was not delegating correctly (was using `null` as
parent loader) so I had to fix that.

I'm not 100% sure that this jextract/type extra annotation makes sense - at the end of the day it is now only used
inside tests (as it makes layout comparison easier - since layouts might have additional attributes such as name and
such).

So I'm open to suggestion on whether we want to keep it or not - I believe that tests which look for specific layouts
should instead probably check for specific types instead. Anyway, we can also start from this cleanup, and take care of
these issues later; the important thing is that, with this rewrite we are finally able to cleanup the ABI constants the
way we wanted.

-------------

Commit messages:
 - Remove trailing whitespaces
 - All jextract tests pass
 - Fixed jextract sources
 - Removed unsigned ABI constants
 - First refactoring of constants - all ABI tests pass

Changes: https://git.openjdk.java.net/panama-foreign/pull/141/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/141/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-JDK-8244270
  Stats: 2779 lines in 50 files changed: 278 ins; 819 del; 1682 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/141.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/141/head:pull/141

PR: https://git.openjdk.java.net/panama-foreign/pull/141