COFF support for Win32

As this comes up in the NG now and then, these are the compiler patches to build COFF files for Win32.

- use command line option -m32coff
- it defines versions CRuntime_DigitalMars and CRuntime_Microsoft (and CRuntime_GNU) as it is no longer possible to detect the C runtime from the OS. This can also replace all the similar adhoc versionings in druntime/phobos.
- C++ interfacing isn't adjusted yet, the rest of the test suite seems to succeed

The corresponding druntime and phobos changes are here (mostly dealing with the CRuntime versions):

https://github.com/rainers/druntime/tree/coff32
https://github.com/rainers/phobos/tree/coff32

If there is a chance of this getting merged, I can make them pull requests, too.
This header shouldn't be leaking into the frontend.  I think the usual way is to make our own types and translate in glue/backconfig?