sched/clock/clock_systimer.c: Change the way that the 64-bit time is sampled....

sched/clock/clock_systimer.c:  Change the way that the 64-bit time is sampled.  Previously, we disabled interrupts before sampling the 64-bit timer since the uint64_t access is not atomic on most CPUs.  However, disabling (local) interrupts does not work in the SMP case.  In that case, the timer interrupt will be running on only one of the CPUs; disabling interrupts on a different CPU will provide no protection from timer rollover.  To work around this, logic was added that samples 64-bit timer is sampled twice and if 32-bit rollover was detected between samples, then loops until there is no rollover.