RFR: 8215097: Do not create NonJavaThreads before BarrierSet

[Kim Barrett]

Please review this change to move the construction of some work gang
threads by G1 and CMS to after they've created the barrier set.  This
allows the removal of some bootstrapping code needed to support that
construction order.  There isn't any requirement for the old order,
it seems to just be a historical artifact.

CR:
https://bugs.openjdk.java.net/browse/JDK-8215097

Webrev:
http://cr.openjdk.java.net/~kbarrett/8215097/open.00/

Testing:
mach5 tier1-5