Introduce GCThreadLocalData to abstract GC-specific data belonging to a thread

As part of the effort to make GCs more pluggable, the G1-specific data in JavaThread should be moved out into a more appropriate abstraction.

This is the second step (building on JDK-8201316), which introduces GCThreadLocalData, an opaque data area in Thread. Each GC is free to decide the internal structure and contents of this area.

With this in place, we can remove G1's thread-local SATB queue and dirty card queue from JavaThread and instead let G1 use the data area provided by GCThreadLocalData to store these.