RFR: 8143014: Access PtrQueue member offsets through derived classes

[Kim Barrett]

Please review this cleanup of the API for accessing member offset
information for SATBMarkQueue and DirtyCardQueue.  Clients now access
the specific queue class information.  In addition, JVMCI now uses the
same API for obtaining the information as is used by other compilers
and interpreters.

Also moved the vmStruct declarations for PtrQueue and members to
vmStructs_g1.hpp, as that class is G1-specific.

CR:
https://bugs.openjdk.java.net/browse/JDK-8143014

Webrev:
http://cr.openjdk.java.net/~kbarrett/8143014/webrev.00/

Testing:
JPRT