Support dates prior to Unix Epoch on Linux

This changes the `st_(a|m|c)time` fields on Linux to
`i64` from `u64` to support times before Unix Epoch.

Before this change, downstream crates would see `i64`
times that have been cast to `u64`, which is unexpected
as it then represents a time unimaginably far in the future.

src/backend/libc/fs/types.rs

@@ -986,11 +986,11 @@ pub struct Stat {
     pub st_size: i64,
     pub st_blksize: u32,
     pub st_blocks: u64,
-    pub st_atime: u64,
+    pub st_atime: i64,
     pub st_atime_nsec: u32,
-    pub st_mtime: u64,
+    pub st_mtime: i64,
     pub st_mtime_nsec: u32,
-    pub st_ctime: u64,
+    pub st_ctime: i64,
     pub st_ctime_nsec: u32,
     pub st_ino: u64,
 }

src/backend/linux_raw/fs/types.rs

@@ -653,11 +653,11 @@ pub struct Stat {
     pub st_size: i64,
     pub st_blksize: u32,
     pub st_blocks: u64,
-    pub st_atime: u64,
+    pub st_atime: i64,
     pub st_atime_nsec: u32,
-    pub st_mtime: u64,
+    pub st_mtime: i64,
     pub st_mtime_nsec: u32,
-    pub st_ctime: u64,
+    pub st_ctime: i64,
     pub st_ctime_nsec: u32,
     pub st_ino: u64,
 }