RFR: 8342963: TestLargeStub::testUpcall doesn't test upcalls stubs

[Jorn Vernee]

We were made aware of the fact the `TestLargeStub::testUpcall` was not actually calling `Linker::upcallStub`. This PR addresses that issue.

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Commit messages:
 - Call upcallStub method in TestLargeStub::upcallStub

Changes: https://git.openjdk.org/jdk/pull/21691/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21691&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8342963
  Stats: 15 lines in 1 file changed: 12 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/21691.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21691/head:pull/21691

PR: https://git.openjdk.org/jdk/pull/21691