RFR (preliminary): JDK-8056999 Make hotspot builds less verbose on default log level

[Magnus Ihse Bursie]

Even in the default log level ("warn"), hotspots builds are extremely 
verbose. With the new jigsaw build system, hotspot is build in parallel 
with the jdk, and the sheer amount of hotspot output makes the jdk 
output practically disappear.

This fix will make the following changes:
* When hotspot is build from the top dir with the default log level, all 
repetetive and purely informative output is hidden (e.g. names of files 
compiled, and the "INFO:" blobs).
* When hotspot is build from the top dir, with any other log level 
(info, debug, trace), all output will be there, as before.
* When hotspot is build from the hotspot repo, all output will be there, 
as before.

Note! This is a preliminary review -- I have made the necessary changes 
for Linux only. If this fix gets thumbs up, I'll continue and apply the 
same pattern to the rest of the platforms. But I didn't want to do all 
that duplication until I felt certain that I wouldn't have to change 
something major. The changes themselves are mostly trivial, but they are 
all over the place :-(.

Bug: https://bugs.openjdk.java.net/browse/JDK-8056999
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8056999-less-verbose-hotspot-builds/webrev.01

/Magnus