cleanup: Set unused fields as private Padding

(backport <rust-lang#4922>)
(cherry picked from commit 0cfa0b2)

Author: 3v1n0

src/unix/aix/mod.rs

@@ -442,7 +442,7 @@ s! {
         pub re_esub: [*mut c_void; 24],
         pub re_map: *mut c_uchar,
         pub __maxsub: c_int,
-        pub __unused: [*mut c_void; 34],
+        __unused: Padding<[*mut c_void; 34]>,
     }
 
     pub struct rlimit64 {

src/unix/bsd/freebsdlike/dragonfly/mod.rs

@@ -417,14 +417,14 @@ s! {
         pub ut_line: [c_char; 32],
         pub ut_host: [c_char; 256],
 
-        pub ut_unused: [u8; 16],
+        ut_unused: Padding<[u8; 16]>,
         pub ut_session: u16,
         pub ut_type: u16,
         pub ut_pid: crate::pid_t,
         ut_exit: exit_status,
         ut_ss: crate::sockaddr_storage,
         pub ut_tv: crate::timeval,
-        pub ut_unused2: [u8; 16],
+        ut_unused2: Padding<[u8; 16]>,
     }
 
     pub struct lastlogx {

src/unix/bsd/netbsdlike/netbsd/mod.rs

@@ -359,7 +359,7 @@ s! {
     }
 
     pub struct uucred {
-        pub cr_unused: c_ushort,
+        cr_unused: Padding<c_ushort>,
         pub cr_uid: crate::uid_t,
         pub cr_gid: crate::gid_t,
         pub cr_ngroups: c_short,

src/unix/hurd/mod.rs

@@ -3330,8 +3330,8 @@ pub const PTHREAD_MUTEX_INITIALIZER: pthread_mutex_t = pthread_mutex_t {
     __shpid: 0,
     __type: PTHREAD_MUTEX_TIMED as c_int,
     __flags: 0,
-    __reserved1: 0,
-    __reserved2: 0,
+    __reserved1: Padding::uninit(),
+    __reserved2: Padding::uninit(),
 };
 pub const PTHREAD_COND_INITIALIZER: pthread_cond_t = pthread_cond_t {
     __lock: __PTHREAD_SPIN_LOCK_INITIALIZER,