RFR: JDK-8217916: Build compare script is not comparing jmods

[Erik Joelsson]

This patch enhances the build compare script to also look at the jmod 
files in the JDK image. This was triggered by JDK-8217826 (linked in 
issue) where we see a rare intermittent comparison failure on Windows 
when looking at file types. That seems to be caused by the file command 
not knowing about jmod files (at least not the versions of the command 
we have).

With this patch, I'm filtering out .jmod files from the file type 
comparison since it seems pointless. Instead I use the jmod tool to list 
the contents of each jmod file and verify that they contain the same 
file names. I don't think there is a point to comparing the contents of 
those files since all the same files will be in the JDK image and will 
be compared anyway. Doing it again would be a waste of time.

Tested manually introducing differences and running the script, as well 
as running the build with COMPARE_BUILD=PATCH= on all relevant platforms.

Bug: https://bugs.openjdk.java.net/browse/JDK-8217916

Webrev: http://cr.openjdk.java.net/~erikj/8217916/webrev.01/index.html

/Erik