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

[Maurizio Cimadamore]

There are two main problems with the way in which ABI-dependent constants are dealt with:

* the SystemABI.Type enum abstraction is rarely used - code just prefers to go with layouts constants in MemoryLayouts
* the current enum doesn't scale to model platform-dependent ABI types, so it just has to work with the set of types that
  are supported in all platforms

This patch rearranges things a bit. First, it removes the SystemABI.Type abstraction, and associated
SystemABI::layoutFor method. Then, it moves all ABI layout constants from MemoryLayouts to SystemABI. Instead of using
the enum value as a classification value for the ABI, each ABI constant holder defines its own classification
constants. So there is a SystemABI.Win64.ArgumentClass, and a SystemABI.SysV.ArgumentClass and so on and so forth. A
notable fact is that the set of classification values that are required by the ABI is much, much smaller than the set
of "interesting" C types.

I then adjusted the implementation code not to no longer depend on the SystemABI.Type enum; while doing so I realized
(suggestion from Jorn) that the public ArgumentClass enums would actually, both in Windows and AArch be enough to also
implement the internal classification system. On SysV, since the classification is quite convoluted, there is a number
of 'synthetic' classification classes that are only introduced when classifying struct (e.g. SSE_UP) which would be bad
to expose outside - but we need these classes to keep the classification algorithm going. So, I moved ArgumentClassImpl
under the sysv folder and kept the code as it was.

The resulting code is, I think more direct, and more scalable. We can keep adding layouts to SystemABI - both
platform-independent and platform-dependent ones - they are just different sets of constants.

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

Commit messages:
 - Fix white spaces
 - * Simplify away some ArgumentClassImpl classes which were not used
 - * Remove SystemABI.Type

Changes: https://git.openjdk.java.net/panama-foreign/pull/97/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/97/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-JDK-8242127
  Stats: 1209 lines in 24 files changed: 438 ins; 677 del; 94 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/97.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/97/head:pull/97

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