Request for Review(xs) - 8155263: DisableStartThread should not be applied to GC worker threads

[Jon Masamitsu]

https://bugs.openjdk.java.net/browse/JDK-8155263

The flag DisableStartThread is defined for Java threads

   develop(bool, DisableStartThread, false, \
           "Disable starting of additional Java threads " \
           "(for debugging only)")

but was being applied to GC threads in one case.  I removed
that use.

Tested by comparing the GC threads before and after the
change to verify that the concurrent GC threads were running.

Using +DisableStartThread on the command line cause the Java
threads not to run and under those circumstances a safepoint
is not reached (Java threads never get there).  Any of the test
programs I tried hung at the first GC.

Thanks.

Jon