RFR: 7903896: JMH: Trim down String literals in generated code

[Aleksey Shipilev]

It is apparent in the heap dump I got when investigating [JDK-8345302](https://bugs.openjdk.org/browse/JDK-8345302) that javac compilation of JDK benchmarks corpus takes significant memory holding String literals referenced in JMH generated code. About 0.6% of them are the ones from Blackhole challenges and internal exceptions.

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jmh/pull/139/files
  Webrev: https://webrevs.openjdk.org/?repo=jmh&pr=139&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903896
  Stats: 13 lines in 3 files changed: 3 ins; 0 del; 10 mod
  Patch: https://git.openjdk.org/jmh/pull/139.diff
  Fetch: git fetch https://git.openjdk.org/jmh.git pull/139/head:pull/139

PR: https://git.openjdk.org/jmh/pull/139