tst-clone3[-internal].c: Add _Atomic to silence Clang

tst-clone3[-internal].c: Add _Atomic to silence Clang

Add _Atomic to futex_wait argument and ctid in tst-clone3[-internal].c to
silence Clang error:

../sysdeps/unix/sysv/linux/tst-clone3-internal.c:93:3: error: address argument to atomic operation must be a pointer to _Atomic type ('pid_t *' (aka 'int *') invalid)
   93 |   wait_tid (&ctid, CTID_INIT_VAL);
      |   ^         ~~~~~
../sysdeps/unix/sysv/linux/tst-clone3-internal.c:51:21: note: expanded from macro 'wait_tid'
   51 |     while ((__tid = atomic_load_explicit (ctid_ptr,                     \
      |                     ^                     ~~~~~~~~
/usr/bin/../lib/clang/19/include/stdatomic.h:145:30: note: expanded from macro 'atomic_load_explicit'
  145 | #define atomic_load_explicit __c11_atomic_load
      |                              ^

Signed-off-by: H.J. Lu <hjl.tools@gmail.com>
Reviewed-by: Sam James <sam@gentoo.org>