s390: Use generic lowlevellock-futex.h

      * sysdeps/unix/sysv/linux/s390/lowlevellock.h: Include
      <sysdeps/nptl/lowlevellock.h> and remove macros and
      functions that are now defined there.
      (SYS_futex): Remove.
      (lll_compare_and_swap): Remove.
      * sysdeps/s390/bits/atomic.h (atomic_exchange_acq): Define.