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

[Erik Joelsson]

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

/Erik