[foreign-memaccess+abi] RFR: 8261972: CLinker.C_VA_LIST layout does not match platform layout for va_list

[Maurizio Cimadamore]

This change removes the `CLinker::VA_LIST` layout. This layout was just an alias for a pointer layout, and was not the correct layout of a `VaList` in given platform. In fact, in some platforms a `VaList` is defined as a one-array element, whereas in other platforms it is defined as a pointer. In all cases, a VaList is always passed to a function by reference, so using `C_POINTER` as a layout, and using `VaList::address` is enough to satisfy the downcall/upcall use cases. Storing valists inside struct fields is a different topic, but it's such a rare/corner/odd use case that I don't think we should distort the API because of that.

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Commit messages:
 - Initial push

Changes: https://git.openjdk.java.net/panama-foreign/pull/567/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=567&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261972
  Stats: 93 lines in 9 files changed: 27 ins; 32 del; 34 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/567.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/567/head:pull/567

PR: https://git.openjdk.java.net/panama-foreign/pull/567