Add private field.
* sysdeps/unix/sysv/linux/lowlevelbarrier.sym: Add PRIVATE definition.
* pthread_barrier_init.c: Set private flag if pshared and private
futexes are supported.
* sysdeps/unix/sysv/linux/i386/i486/pthread_barrier_wait.S: Use
private field in futex command setup.
* sysdeps/unix/sysv/linux/x86_64/pthread_barrier_wait.S: Likewise.