Rename DigitalMarsWin32 / DigitalMarsWin64 to DMC_RUNTIME / MSVC_RUNTIME.

This has the advantages:
- DMC_RUNTIME is used in Phobos, too.
- I don't have to write

    version(LDC) version(Win64) version = DigitalMarsWin64;

which I find confusing.
Merge pull request #853 from redstar/msvcrt

Rename DigitalMarsWin32 / DigitalMarsWin64 to DMC_RUNTIME / MSVC_RUNTIME