Open the file as write before trying to flush it

This should be enough and shouldn't require append(true) since we're not
explicitly writing anything so we're not flushing it so we've no risk of
overwriting it

src/librustc_trans/back/link.rs

@@ -894,7 +894,7 @@ fn exec_linker(sess: &Session, cmd: &mut Command, out_filename: &Path, tmpdir: &
 
         if let &Ok(ref out) = command_output {
             if out.status.success() {
-                if let Ok(of) = fs::File::open(out_filename) {
+                if let Ok(of) = fs::OpenOptions::new().write(true).open(out_filename) {
                     of.sync_all()?;
                 }
             }