[foreign-jextract] RFR: JDK-8244601: Cleanup support for upcall handles (jextract part)

[Maurizio Cimadamore]

This small patch fixes jextract to sync up with latest changes on SystemABI::upcallHandle.

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Commit messages:
 - Fixup jextract to sync up with upcall ABI changes

Changes: https://git.openjdk.java.net/panama-foreign/pull/155/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/155/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-JDK-8244601
  Stats: 12 lines in 5 files changed: 0 ins; 4 del; 8 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/155.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/155/head:pull/155

PR: https://git.openjdk.java.net/panama-foreign/pull/155