RFR(XXS) 8222188: Shenandoah: Adjust Shenandoah work gang types

[Zhengyu Gu]

This is a cleanup in preparation for concurrent class unloading.

Bug: https://bugs.openjdk.java.net/browse/JDK-8222188
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8222188/webrev.00/index.html

Test:
   hotspot_gc_shenandoah on Linux 64 (fastdebug and release)

Thanks,

-Zhengyu