RFR: JDK-8058797: Building with sjavac broken after JDK-8058118

[Erik Joelsson]

In JDK-8058118, a small java tool is compiled very early in the make 
process. This compilation cannot use sjavac since it's done with the 
boot javac. The fix is simple. Just add DISABLE_SJAVAC := true to the 
macro call. I also added a comment documenting this already existing 
feature.

Bug: https://bugs.openjdk.java.net/browse/JDK-8058797
Webrev: http://cr.openjdk.java.net/~erikj/8058797/webrev.root.01/
/Erik