[CMake] Don't LTO optimize targets on Darwin, but only if its not ThinLTO

This is just a workaround. Pass the `-mllvm,-O0` link flags only if its
not ThinLTO. Doing that with ThinLTO currently results in an error:

```
Remaining virtual register operands
UNREACHABLE executed at .../llvm/lib/CodeGen/MachineRegisterInfo.cpp:209!
```