Oh, interesting... Ex-Transitive guy here!
A quick take on GCCJIT and QEMU TCG is that they're very basic for the purpose of replacing the UAE JIT and GCC is just heavyweight. If the motivation was to simply compile and run under x86_64 then sticking to the same i386 subset could be far more manageable as there are only a few common problems here such as default size of data pushed to stack, removed instructions (maybe not even applicable); far from a rewrite to port the ABI. I've taken a look at the UAE sources and it's fairly parseable to me but alas I'm likely encumbered.
