JDK 12 RFR of JDK-8207055 : Make javac -help output for -source and -target more informative

[joe darcy]

Hello,

Please review the changes to implement

     JDK-8207055 : Make javac -help output for -source and -target more 
informative
     http://cr.openjdk.java.net/~darcy/8207055.1/

Basically the -help output for -source and -target is augmented to 
include a concatenation of the supported values for those options. It 
would be possible to write the concatenation in a more stream-y manner, 
but I think the loops are sufficient.

Thanks,

-Joe