Merge pull request rust-lang#4298 from hermit-os/hermit-st_size

hermit: make `stat::st_size` signed

Author: tgross35

src/hermit.rs

@@ -100,7 +100,7 @@ s! {
         pub st_uid: u32,
         pub st_gid: u32,
         pub st_rdev: u64,
-        pub st_size: u64,
+        pub st_size: i64,
         pub st_blksize: i64,
         pub st_blocks: i64,
         pub st_atim: timespec,