RFR[XS] 8210246 NMTUtil::_memory_type_names should be in sync with MemoryType

[Ioi Lam]

https://bugs.openjdk.java.net/browse/JDK-8210246
http://cr.openjdk.java.net/~iklam/jdk12/8210246-memory_type_names.v01/

This is a small clean up:

MemoryType and _memory_type_names should be in sync, but they were
coded independently in 2 files, and it's possible to change one while
forgetting to change the other.

The fix is, as always, some fancy macros.

Thanks
- Ioi