RFR: 8343771: Some FFM benchmarks are broken

[Per Minborg]

This PR fixes some regressions in the FFM benchmarks introduced by https://bugs.openjdk.org/browse/JDK-8332744.

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Commit messages:
 - Add imports to FFM benchmarks

Changes: https://git.openjdk.org/jdk/pull/21963/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21963&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8343771
  Stats: 61 lines in 21 files changed: 25 ins; 0 del; 36 mod
  Patch: https://git.openjdk.org/jdk/pull/21963.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21963/head:pull/21963

PR: https://git.openjdk.org/jdk/pull/21963