JDK 12 RFR of build portions of JDK-802856: Remove javac support for 6/1.6 source and target values

[joe darcy]

Hello,

Please review the build portions of

      JDK-802856: Remove javac support for 6/1.6 source and target values
      http://cr.openjdk.java.net/~darcy/8028563.4/

The langtools aspects of the change are out for review on compiler-dev [1].

I've checked with Jan Lahoda who implemented much of the --release 
feature and he agreed editing the filed marked as generated was the most 
reasonable way to drop 6 as a platform at this time.

Thanks,

-Joe


[1] 
http://mail.openjdk.java.net/pipermail/compiler-dev/2018-July/012167.html