[JENKINS-755] Describe the built-in JDK as '(System)'.

As suggested in
https://issues.jenkins-ci.org/browse/JENKINS-755?focusedCommentId=206175&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-206175
this avoids confusion where users incorrectly think they're getting a
JDK they've listed in the configuration page.