If you are running it in a chroot, or can ssh into it, then
Could you post the output of
emerge --info ; profile-config show
And also the output of
emerge -NavDu --backtrack=50 -p @world
The '--backtrack=50' sometimes is all thats needed to fix the issue. But it's likely we will need to be a bit more involved.
If your stuck without network connectivity on the machine, or the above options are problematic for some other reason,
Run
emerge --info >/tmp/PortageInfo ; profile-config show >>/tmp/PortageInfo;
emerge -NavDu --backtrack=50 -p @world > /tmp/EmergeInfo
And th