RFR: JDK-8234689 facilitate writing additional custom attributes in a class file

[Jonathan Gibbons]

Please review a small change to javac to permit friendly clients to 
inject additional attributes into a class file.

This supersedes and generalizes an existing mechanism which is 
difficult/impractical to use, especially
in current versions of javac where split packages are strongly discouraged.

-- Jon

JBS: https://bugs.openjdk.java.net/browse/JDK-8234689
Webrev: http://cr.openjdk.java.net/~jjg/8234689/webrev.00/