RFR(s): 8191864: Provide a public destructor for WorkGang

[Per Liden]

Hi,

WorkGang's destructor is currently declared private, meaning a WorkGang 
can't be deleted, instantiated as a value or placed on the stack. This 
patch adds a public destructor. I also added a protected destructor on 
AbstractWorkGang to guard against attempts to delete using a 
AbstractWorkGang pointer, which would require a virtual destructor.

Webrev: http://cr.openjdk.java.net/~pliden/8191864/
Bug: https://bugs.openjdk.java.net/browse/JDK-8191864

/Per