Everything is in the title.

If you run an old kernel (it might be the case with xen-sources), you should not upgrade to coreutils-8.7. It will break at least the touch binary, resulting in portage’s emerge being broken, with the following error for every emerge: touch: setting times of `/var/tmp/portage/sys-apps/portage-’: No such file or directory, even though the file exists.

If you were unlucky and applied the update, here is a workaround, as root :
mv /bin/touch /bin/oldtouch
echo '#!/bin/sh' > /bin/touch
echo 'echo -n >> "$1"' >> /bin/touch
chmod +x /bin/touch

After that, you might want to use the following or check the existing patch in the bugzilla:
echo '>=sys-apps/coreutils-8.7' >> /etc/portage/package.mask
emerge -1av "<coreutils-8.7"
rm /bin/oldtouch

Here is the bugzilla entry: https://bugs.gentoo.org/show_bug.cgi?id=348640. To this day, even though there is a patch and the bug entry has been marked as resolved, it’s not really in the mainstream…