COFF support for Win32

These are the changes for COFF support for Win32. It uses CRuntime_DigitalMars/CRuntime_Microsoft instead of Win32/Win64 where appropriate. Most of the exisiting adhoc version identifiers are not yet replaced.

There are some places where it abuses "win64" in the filename instead of the more correct "winms", namely win64.mak and sections_win64.d. If corrected the win32 versions should be renamed to "windm" or similar, too.

compile from the VS command line with
```
make -f win64.mak MODEL=32mscoff "CC=\"%VCINSTALLDIR%\bin\cl.exe\"" target unittest
```