RFR: 8338570: sun/font/HBShaper - mismatch in return type of FFM upcall function description and native invocation

[Phil Race]

Fix a couple of function signature mismatches.
No regression test viable that I can see.

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Commit messages:
 - 8338570

Changes: https://git.openjdk.org/jdk/pull/21588/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21588&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338570
  Stats: 8 lines in 3 files changed: 0 ins; 2 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/21588.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21588/head:pull/21588

PR: https://git.openjdk.org/jdk/pull/21588