Fix return type of FileAttr methods on AIX target
At some point it seems `SystemTime::new` changed from returning `SystemTime` to `io::Result<SystemTime>`. This seems to have been addressed on other platforms, but was never changed for AIX.
This was caught by running
```
python3 x.py build --host x86_64-unknown-linux-gnu --target powerpc64-ibm-aix
```