RFR: JDK-8140312: Enable new sjavac server only mode in jdk build

[Erik Joelsson]

In JDK-8135131, sjavac added a thin server mode, in which only the 
server part of sjavac is used for running plain javac. This patch adds a 
configure option to enable this mode of running to the jdk build. Later, 
if it seems to work well, we will consider making it default.

On my workstation (linux) running the default make target went from 
about 4m55s to 4m05s when activating this.

Bug: https://bugs.openjdk.java.net/browse/JDK-8140312
Webrev: http://cr.openjdk.java.net/~erikj/8140312/webrev.01/index.html

/Erik