RFR: 8214056: Allow the GC to attach context information to CompiledMethod

[Erik Österlund]

Hi,

A GC might want extra GC-related information attached to a CompiledMethod.
I propose to introduce a type-erased _gc_data member, with accessors.
This allows ZGC to stick in some data we need to keep track of 
concurrent class unloading.

Webrev:
http://cr.openjdk.java.net/~eosterlund/8214056/webrev.00/

Bug ID:
https://bugs.openjdk.java.net/browse/JDK-8214056

Thanks,
/Erik