A functioning LLVM backend does exist (https://github.com/espressif/llvm-project/issues/4) and appears to be making very slow progress towards being merged. A quick search shows that it works for Rust. I suspect (but don't know) that it might work for Terra as well.
I hadn't realized LLVM mainline doesn't support Xtensa. I'm surprised.
D does support Xtensa via LDC (https://forum.dlang.org/thread/[email protected]...). It looks like GDC also nearly supports it, requiring only a minor patch at present.
A functioning LLVM backend does exist (https://github.com/espressif/llvm-project/issues/4) and appears to be making very slow progress towards being merged. A quick search shows that it works for Rust. I suspect (but don't know) that it might work for Terra as well.
There's also the LLVM C backend (https://github.com/JuliaComputingOSS/llvm-cbe) but I've no idea how efficient such an approach is when applied to real world embedded tasks.