NMT: Make mtGCCardSet the subcategory of mtGC

JDK-8017163 introduced a new category, mtGCCardSet in NMT tracking. This makes logging look rather weird, and technically miscounts the space taken by GC-specific datastructures:

- GC (reserved=142426KB, committed=142426KB)
                            (malloc=89554KB #3658)
                            (mmap: reserved=52872KB, committed=52872KB)
 
- GCCardSet (reserved=128KB, committed=128KB)
                            (malloc=128KB #1539)

We can instead recast it as the subcategory of mtGC. It would be similar to mtThreadStacks that are accounted separately, but reported as part of mtThread.

- GC (reserved=142584KB, committed=142584KB)
                            (card sets: 128KB)
                            (malloc=89583KB #3780)
                            (mmap: reserved=52872KB, committed=52872KB)