Make it clear that the CPU listed on the download page is the host, not target

There [was some confusion](http://forum.dlang.org/post/tzutuouuvyzmziajsnyb@forum.dlang.org) after pull #593, which probably will require some documentation on Win64 also.  I'd like to also add somewhere that all dmd binaries support both i386 and x86_64 targets, but I'm not sure where to put it on the page.  It's unclear if gdc is only listing host CPUs or target CPUs also, so I left that one the way it is.