[D-runtime] druntime fixes again break Windows DLL support

[Walter Bright]

I deleted all the .di files. Still fails the test suite:

..\dmd -c mydll
mydll.d(12): Error: module dll_helper is in file 'core\dll_helper.d' which 
cannot be read
import path[0] = \cbx\mars\phobos
import path[1] = \cbx\mars\druntime\import

--- errorlevel 1

I know it's because core\dll_helper.d is gone, but the tests need to be fixed, 
and I'm not sure what needs to be done with them.