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.
Will do.

I think this is the ugliest part of this PR. I always wondered why the backend configuration is not a struct declared in a header file. Right now, the prototype for out_config_init is duplicated in msc.c making it inconvenient to add another option because other backends will need to be updated, too.