Consistently name gc files containing VM operations

There is some confusion in the way we name files containing GC VM operations, i.e.

./runtime/vm_operations.hpp
./runtime/vm_operations.cpp
./gc/shared/vmGCOperations.cpp
./gc/shared/vmGCOperations.hpp
./gc/cms/vmCMSOperations.cpp
./gc/cms/vmCMSOperations.hpp
./gc/parallel/vmPSOperations.hpp
./gc/parallel/vmPSOperations.cpp
./gc/g1/vm_operations_g1.cpp
./gc/g1/vm_operations_g1.hpp

After some discussion, rename them consistently to <gc>VMOperations.?pp.