RFR: 7903984: Inline upcallHandle method to reduce shared code across multiple generations

[Nizar Benalla]

Please review this patch to inline the upcallHandle method, reducing the amount of shared items across multiple jextract generations.

TIA

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Commit messages:
 - remove whitespace
 - inline method

Changes: https://git.openjdk.org/jextract/pull/279/files
  Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=279&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903984
  Stats: 18 lines in 2 files changed: 8 ins; 8 del; 2 mod
  Patch: https://git.openjdk.org/jextract/pull/279.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/279/head:pull/279

PR: https://git.openjdk.org/jextract/pull/279