WIP: LLVM: Use musttail instead of tail

Trying out musttail instead of tail for LLVM calls.

From the docs:

The optional tail and musttail markers indicate that the optimizers should perform tail call optimization. The tail marker is a hint that can be ignored. The musttail marker means that the call must be tail call optimized in order for the program to be correct. https://llvm.org/docs/LangRef.html#call-instruction

I think mustail is what we would expect for GHC, but let's see what the CI results are...