There are no binaries for Yosemite yet, so that explains why fetching the signature would fail. What I can't explain is why fetching the archive itself would succeed, since it doesn't exist on our servers.
Please clean ld64 and try again. Also please ensure that your DNS servers function properly; see MisbehavingServers for some common errors we see with broken DNS servers.
If none of that helps, you can install with the -s flag (as in sudo port -s install gcc48); this will instruct MacPorts to not even try to get a binary. Or you can accomplish the same thing by editing macports.conf and telling it to always build from source. You can remove this setting once binaries become available for Yosemite. This is probably still several weeks away and will be announced on the MacPorts mailing lists.
Note that gcc47 and earlier do not work on Yosemite yet; see #43978. Use gcc48 or newer for now.
