RFR: 8349238: Some more FFM benchmarks are broken

[Per Minborg]

This PR proposes to fix an Unsafe issue in a benchmark.

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Commit messages:
 - Add usage of Unsafe

Changes: https://git.openjdk.org/jdk/pull/23424/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23424&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8349238
  Stats: 26 lines in 1 file changed: 25 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/23424.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23424/head:pull/23424

PR: https://git.openjdk.org/jdk/pull/23424