RFR: CODETOOLS-7902813: Amend experimental compiler-assisted blackhole support

[Aleksey Shipilev]

JDK-8258619 changes the requirements for compiler-assisted blackholes to be empty static methods. This requires adjustments to JMH Blackhole itself.

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Commit messages:
 - CODETOOLS-7902813: Amend experimental compiler-assisted blackhole support

Changes: https://git.openjdk.java.net/jmh/pull/16/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmh&pr=16&range=00
  Issue: https://bugs.openjdk.java.net/browse/CODETOOLS-7902813
  Stats: 206 lines in 4 files changed: 126 ins; 33 del; 47 mod
  Patch: https://git.openjdk.java.net/jmh/pull/16.diff
  Fetch: git fetch https://git.openjdk.java.net/jmh pull/16/head:pull/16

PR: https://git.openjdk.java.net/jmh/pull/16