Currently Zig lowers `@intToFloat` for f80 incorrectly on non-x86
targets:

```
broken LLVM module found:
UIToFP result must be FP or FP vector
  %62 = uitofp i64 %61 to i128
SIToFP result must be FP or FP vector
  %66 = sitofp i64 %65 to i128
```

This happens because on such targets, we use i128 instead of x86_fp80 in
order to avoid "LLVM ERROR: Cannot select". `@intToFloat` must be
lowered differently to account for this difference as well.