tst-atexit-common.c: Mark _exit_with_flush as noreturn

tst-atexit-common.c: Mark _exit_with_flush as noreturn

Mark _exit_with_flush as noreturn to silence the Clang error on
tst-atexit-common.c:

In file included from tst-atexit.c:22:
../stdlib/tst-atexit-common.c:113:5: error: unannotated fall-through between switch labels [-Werror,-Wimplicit-fallthrough]
  113 |     case 0:  /* Child.  */
      |     ^
../stdlib/tst-atexit-common.c:113:5: note: insert 'break;' to avoid fall-through
  113 |     case 0:  /* Child.  */
      |     ^
      |     break;
1 error generated.

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