RFR: JDK-8205934: Define jdk -source/-target version in version-numbers file

[Tim Bell]

Erik:

> To make it easier to update the major version in the future, we should
> define a make variable for the -source/-target version number used when
> compiling jdk classes and have it be initiated through
> make/common/version-numbers, like all other version numbers. This patch
> does just that.
>
> Bug: https://bugs.openjdk.java.net/browse/JDK-8205934
> <https://bugs.openjdk.java.net/browse/JDK-8205934?filter=-1>
>
> Webrev: http://cr.openjdk.java.net/~erikj/8205934/webrev.01/index.html

Looks good.

Tim