clock_systimer: Replace critical section APIs with spinlock APIs. (64bit only)

If SMP=n or SMP=y && SPINLOCK_IRQ=n, this works in the same way as before.
If SMP=y && SPINLOCK_IRQ=y, performance will be improved.

Signed-off-by: Masayuki Ishikawa <Masayuki.Ishikawa@jp.sony.com>