RFR: 8280088: NMT: Make mtGCCardSet the subcategory of mtGC

[Aleksey Shipilev]

[JDK-8017163](https://bugs.openjdk.java.net/browse/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) 


Additional testing:
 - [x] Linux x86_64 fastdebug `runtime/NMT`
 - [ ] Linux x86_64 fastdebug `tier1`

-------------

Commit messages:
 - Fix

Changes: https://git.openjdk.java.net/jdk/pull/7115/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7115&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8280088
  Stats: 22 lines in 1 file changed: 22 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7115.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7115/head:pull/7115

PR: https://git.openjdk.java.net/jdk/pull/7115