time: Enable get/put_compat_itimerspec64 always

This will aid in enabling the compat syscalls on 32-bit architectures later
on.

Also move compat_itimerspec and related defines to compat_time.h.  The
compat_time.h file will eventually be deleted.

Signed-off-by: Deepa Dinamani <deepa.kernel@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: arnd@arndb.de
Cc: viro@zeniv.linux.org.uk
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-api@vger.kernel.org
Cc: y2038@lists.linaro.org
Link: https://lkml.kernel.org/r/20180617051144.29756-3-deepa.kernel@gmail.com