[foreign-jextract] RFR: 8261906: Improve jextract support for virtual functions

[Maurizio Cimadamore]

This patch add support for function pointer calls. Whenever jextract encounters a function pointer variable (be it a global variable, or a struct field), it will additionally emit code to _call_ the function pointer - by means of a static wrapper around a virtual method handle.

The implementation is relatively straightforward, although there is some duplication in the code that is being emitted. I played with this a bit, to see if duplication could be removed, but I ended up with a single routine to generate wrappers which worked across the axis { virtual, non virtual } x { struct, global } - which in the end I found too complex for my taste (note that, e.g. when we're inside a struct, the logic to get the virtual address depends on what else is generated inside the struct, so there's a lot of ad-hocness).

For these reason I ended up with separate emit function for virtual wrappers (e.g. I did not reuse the code for native functions) - the code which emit virtual function wrappers is also overridden in StructBuilder. My sense is that if we move the code to use text blocks, the duplication will be much less problematic.

In OutputFactory, I cleaned up things a bit - since I realized that support for valist doesn't work - this is now removed. I will file a followup issue to add proper VaList support, both in downcalls and upcalls.

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Commit messages:
 - Simplify code
 - Initial push

Changes: https://git.openjdk.java.net/panama-foreign/pull/456/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=456&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261906
  Stats: 509 lines in 10 files changed: 413 ins; 79 del; 17 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/456.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/456/head:pull/456

PR: https://git.openjdk.java.net/panama-foreign/pull/456