Further to the work and blogs by Xerces and Gary, I forwardported the necessary changesets to IcedTea7 and was able to do a full bootstrap there too:

$ /mnt/builder/shark/openjdk/build/linux-amd64/j2sdk-image/bin/java -version
java version “1.7.0_0-icedtea”
OpenJDK Runtime Environment (IcedTea7 1.10-r05f411adf2f1) (build 1.7.0_0-icedtea-b59)
OpenJDK 64-Bit Shark VM (build 16.0-b03, mixed mode)

Sadly it seems I had to run make twice to achieved this – PR 348 – I hope this is just some minor issue with the version of LLVM.