progress update:
 - binutils-2.22 in ~arch should work fine
 - glibc-2.14.1-r1 in ~arch includes support when "x32" is in MULTILIB_ABIS
 - linux-headers-3.1 includes support when "x32" is in MULTILIB_ABIS
 - you'll still need gcc-4.7 from the toolchain overlay
 - a 3.1 kernel can be obtained here: https://github.com/hjl-tools/linux

the only problem is with bootstrapping.  there is no way atm to live migrate 
between multilibs on your system.  this isn't a new problem ... we've been 
punting all bugs where people try to go from MULTILIB_ABIS="amd64" to "x86 
amd64" on a live system.  same issue applies when adding "x32" to the list.

i've been contemplating how to address this, but so far, it seems like the 
easiest is to post a glibc binpkg which includes all possible ABIs and have 
people install that first.  then they should be able to install gcc/linux-
headers freely and rebuild glibc after that.
-mike