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
```

Author: hallfox

library/std/src/sys/pal/unix/fs.rs

@@ -463,15 +463,15 @@ impl FileAttr {
 #[cfg(target_os = "aix")]
 impl FileAttr {
     pub fn modified(&self) -> io::Result<SystemTime> {
-        Ok(SystemTime::new(self.stat.st_mtime.tv_sec as i64, self.stat.st_mtime.tv_nsec as i64))
+        SystemTime::new(self.stat.st_mtime.tv_sec as i64, self.stat.st_mtime.tv_nsec as i64)
     }
 
     pub fn accessed(&self) -> io::Result<SystemTime> {
-        Ok(SystemTime::new(self.stat.st_atime.tv_sec as i64, self.stat.st_atime.tv_nsec as i64))
+        SystemTime::new(self.stat.st_atime.tv_sec as i64, self.stat.st_atime.tv_nsec as i64)
     }
 
     pub fn created(&self) -> io::Result<SystemTime> {
-        Ok(SystemTime::new(self.stat.st_ctime.tv_sec as i64, self.stat.st_ctime.tv_nsec as i64))
+        SystemTime::new(self.stat.st_ctime.tv_sec as i64, self.stat.st_ctime.tv_nsec as i64)
     }
 }