RFR: 8349343: Add missing copyright messages in FFM benchmarks

[Per Minborg]

This PR proposes to add missing copyright headers to some FFM benchmarks.

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Commit messages:
 - Add copyright headers

Changes: https://git.openjdk.org/jdk/pull/23433/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23433&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8349343
  Stats: 184 lines in 8 files changed: 184 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/23433.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23433/head:pull/23433

PR: https://git.openjdk.org/jdk/pull/23433