RFR: JDK-8198318 Make build comparisons clean again

[Magnus Ihse Bursie]

Over time, small differences has accumulated in the build that makes it 
non-reproducible. This patch addresses some of these, and modifies 
compare.sh to properly handles those that cannot be fixed.

This patch is mainly due to Erik, who wrote most of it. I've mostly 
finished it up.

Bug: https://bugs.openjdk.java.net/browse/JDK-8198318
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8198318-make-build-comparisons-clean-again/webrev.01

/Magnus