Generate the DLL name provides from mingw-crt import libraries
The virtual mingw32(...) and mingw64(...) provides used to be generated
from the names of WINE DLLs. This changes it to generate the list from
mingw-crt import libs instead. The advantage with the new approach is
that we can be sure that the virtual provides match with the actual DLL
names we can end up linking with: we'll be linking with the same stubs
from mingw-crt package that are used for the virtual provides list.