std: fix linux uid_t, use uid_t/gid_t in std.os

- correct uid_t from i32 to u32 on linux
- define uid_t and gid_t for OSes missing definitions
- use uid_t/gid_t instead of plain u32s throughout std.os

lib/std/os/bits/linux/x86_64.zig

@@ -7,6 +7,7 @@
 const std = @import("../../../std.zig");
 const pid_t = linux.pid_t;
 const uid_t = linux.uid_t;
+const gid_t = linux.gid_t;
 const clock_t = linux.clock_t;
 const stack_t = linux.stack_t;
 const sigset_t = linux.sigset_t;
@@ -523,8 +524,8 @@ pub const Stat = extern struct {
     nlink: usize,
 
     mode: u32,
-    uid: u32,
-    gid: u32,
+    uid: uid_t,
+    gid: gid_t,
     __pad0: u32,
     rdev: u64,
     size: off_t,