Add source 12 and target 12 to javac

Summary

Add -source and -target options to javac corresponding to JDK 12.

Problem

Part of starting development on a new JDK is adding .new -source and -target options to javac. These options are used in the compilation of the JDK sources.

Solution

Add support for javac -source 12 ... and javac -target 12 ....

The new value of -source/-target is also made the default.

Note: following the precedent set in JDK 11, "1.12" is not treated as an alias for "12".

Specification

No update is needed in javac --help; the conceptual man page should reflect the increased range of -source/-target values.