RFR: 8257848: -XX:CompileCommand=blackhole,* should be diagnostic

[Aleksey Shipilev]

This follows up on retroactive CSR review comments (JDK-8257827). As the result of discussion, it is decided to demote the "blackhole" compile command to diagnostic. Examples:

$ build/linux-x86_64-server-release/images/jdk/bin/java -Xcomp -XX:CompileCommand=quiet -XX:CompileCommand=blackhole,java/lang/Object.toString
OpenJDK 64-Bit Server VM warning: Blackhole compile option is diagnostic and must be enabled via -XX:+UnlockDiagnosticVMOptions

$ build/linux-x86_64-server-release/images/jdk/bin/java -Xcomp -XX:+UnlockDiagnosticVMOptions -XX:CompileCommand=quiet -XX:CompileCommand=blackhole,java/lang/Object.toString
OpenJDK 64-Bit Server VM warning: Blackhole compile option only works for methods with void type: java.lang.Object.toString()Ljava/lang/String;

New test verifies the locking/unlocking of the command.

Testing:
 - [x] ad-hoc `compiler/blackhole` test
 - [x] validation runs with JMH
 - [ ] runs in `tier1` (needs #1654 to be merged)

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

Commit messages:
 - 8257848: -XX:+CompilerCommand=blackhole,* should be diagnostic

Changes: https://git.openjdk.java.net/jdk/pull/1674/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1674&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8257848
  Stats: 320 lines in 4 files changed: 216 ins; 101 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1674.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1674/head:pull/1674

PR: https://git.openjdk.java.net/jdk/pull/1674