RFR: 8220610: Make CollectedHeap nmethod functions pure virtual

[Stefan Karlsson]

Hi all,

I suggest that we make all CollectedHeap nmethod functions pure virtual.

http://cr.openjdk.java.net/~stefank/8220610/webrev.01/
https://bugs.openjdk.java.net/browse/JDK-8220610

This forces GC implementers to actively decide if they need extra 
handling of nmethods, and oops in nmethods.

Thanks,
StefanK