RFR: 7902762: JMH: Use "blackhole" compiler command [v4]

[Aleksey Shipilev]

> JMH can be enabled to use the "blackhole" compile command from [JDK-8252505](https://bugs.openjdk.java.net/browse/JDK-8252505).

Aleksey Shipilev has updated the pull request incrementally with two additional commits since the last revision:

 - Copyrights in new file
 - Print experimental support option

-------------

Changes:
  - all: https://git.openjdk.java.net/jmh/pull/8/files
  - new: https://git.openjdk.java.net/jmh/pull/8/files/707bc782..ab9627fe

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jmh&pr=8&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jmh&pr=8&range=02-03

  Stats: 124 lines in 5 files changed: 66 ins; 23 del; 35 mod
  Patch: https://git.openjdk.java.net/jmh/pull/8.diff
  Fetch: git fetch https://git.openjdk.java.net/jmh pull/8/head:pull/8

PR: https://git.openjdk.java.net/jmh/pull/8