RFR: 8257848: -XX:CompileCommand=blackhole, * should be diagnostic [v4]

[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)

Aleksey Shipilev has updated the pull request incrementally with one additional commit since the last revision:

  Check blackhole unlocking at startup

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/1674/files
  - new: https://git.openjdk.java.net/jdk/pull/1674/files/b2da3cdf..cbc07869

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=1674&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=1674&range=02-03

  Stats: 10 lines in 1 file changed: 6 ins; 4 del; 0 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