RFR: JDK-8138864 Build with nice to lower UI impact

[Magnus Ihse Bursie]

Since building the OpenJDK is a heavy operation, the build system tries 
to make use as of much of the system resources as possible and reasonable.

However, on most machines except extremely high-end, this means that few 
resources are available for other processes. On a developer machine, 
building means that other work can get laggy and have poor response times.

There is a simple remedy for this! Using "nice" to lower the priority 
for the build process, so the build will wait for normal UI processes. A 
single "nice" when handling the parallel targets in Init.gmk is sufficient.

A potential problem could be if this affects performance on dedicated 
build servers. I have checked this on Oracle's internal build system, 
and there were no measurable regressions. This also stands to reason, 
since if there would be, the build server would be running other 
processes competing with the build. On a correctly setup build server, 
this should not be the case. And if such processes existed, it would be 
bad anyway.

Bug: https://bugs.openjdk.java.net/browse/JDK-8138864
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8138864-make-nice-priority/webrev.01

/Magnus