Consider switching from musl to llvm-libc?

llvm-libc is improving a lot it has a few advantages. The primary would be that we can upstream our changes and our backend code. It would also mean that we get libc from the same place we get all our other standard libraries.

We should probably do a small experiment. e.g. try using just the `string.h` functions and then discuss more once we have the results of that.