The MIPS target of LLVM generates FPU instructions by default and there is no way to specify soft float in MIPS triple.
I enable soft float by default because I think it works on most MIPS CPUs.

I don't know what is the best way to handle this option.
Is it acceptable to hardcode softfp option in ```LLVMRustCreateTargetMachine``` like this
```
if (Trip.getArch() == Triple::mips) {
    Options.UseSoftFloat = true;
    Options.FloatABIType = FloatABI::Soft;
}
```