Put all .o files and the generated dmd file in a separate directory

This cleans up the mess of having .o files and the dmd binary itself in the same place as the source files. I'm also considering moving the generated .c files into the same dir.

Aside from the cleanliness, another advantage of the proposed scheme is that one can build files for MODEL=32 and MODEL=64 without weird errors and without needing to clean the build first.