RFR: 8238951: Update jextract after JDK-8238226

[Jorn Vernee]

Hi,

PR #15 removed variadic information from FunctionDescriptor. We need to update it's uses in jextract as well.

This also includes 2 small fixes to make the tests pass on Windows:
- Missing EXPORT in makePoint declaration
- library name on Windows should be 'libclang' not 'clang'
(see https://github.com/openjdk/panama-foreign/pull/17/commits/4407d800736db2296ac2784f9a7768fdf5a1b561)

Jorn

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Commits:
 - e60d6c18: Update FunctionDescriptor uses
 - 4407d800: Makes test run on Windows

Changes: https://git.openjdk.java.net/panama-foreign/pull/17/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/17/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8238951
  Stats: 357 lines in 6 files changed: 10 ins; 1 del; 346 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/17.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/17/head:pull/17

PR: https://git.openjdk.java.net/panama-foreign/pull/17