fix typo in the creation of OpenOption

Due to this typo we have to build a workaround for issue
hermitcore/libhermit-rs#191.

RustyHermit is a tier 3 platform and backward compatibility does
not have to be guaranteed.

library/std/src/sys/hermit/fs.rs

@@ -202,7 +202,7 @@ pub fn new() -> OpenOptions {
             create: false,
             create_new: false,
             // system-specific
-            mode: 0x777,
+            mode: 0o777,
         }
     }