Y2038: add function __clock_gettime64

Y2038: add function __clock_gettime64

* include/time.h: Declare __clock_gettime64().
* ntpl/pthread_clock_gettime.c: Add __pthread_clock_gettime64().
* ntpl/pthread_clock_gettime.c: Make __pthread_clock_gettime()
  a wrapper around __pthread_clock_gettime64().
* sysdeps/unix/clock_gettime.c (hp_timing_gettime): Use struct
  __timespec64.
* sysdeps/unix/clock_gettime.c (realtime_gettime): Likewise.
* sysdeps/unix/clock_gettime.c: Add __clock_gettime64().
* sysdeps/unix/clock_gettime.c: Make __clock_gettime() a
  wrapper around __clock_gettime64().
* sysdeps/unix/sysv/linux/clock_gettime.c: Add 64-bit-time syscall
  support.