Clarify which CPUs are supported by DMD on which platform

The binaries in the ZIP file on the download page don't run on x86 **and** x86_64 for all architectures, see here:
http://forum.dlang.org/thread/ezfhuuruflirphpyqxly@forum.dlang.org#post-lne8b0:2413r9:241:40digitalmars.com