I didn't expect the created 'as'-binary to be linking to the temp directory in /var/tmp/portage
I indeed interrupted the compiling process after the 'as' binary was done compiling, it would be all you need...
As a next trial, i created a full binary package for you, sent to your mailbox.
Here's how to use it, in case you don't already know:
1) put the file in /usr/portage/packages/All/ (create directories if necessary, you probably have to if you haven't used binary packages before)
2) run emerge -K binutils
It should emerge the binary package over your current binutils installation. After that it should work and i suggest the first thing to do afterwards is to re-merge binutils to compile your own binutils version against your own CFLAGS, USE-list etc
There may be one problem tough; the binary package i created is using /usr/i686-pc-linux-gnu/ (i have a singlecore P4 CPU) to install it's binaries while yours is /usr/x86_64-pc-linux-gnu/. As a consequence you may be required to do some moving afterwards. You could try to create a symlink from /usr/i686-pc-linux-gnu/ to /usr/x86_64-pc-linux-gnu/ before emerging the binary package.