kernel: Protect cpu_ent::active_time with sequential lock

atomic_{get, set}64() are problematic on architectures without 64 bit
compare and swap.
Also, using sequential lock instead of atomic access ensures that
any reads from cpu_ent::active_time won't require any writes to shared
memory.

headers/private/kernel/cpu.h

@@ -56,6 +56,7 @@ typedef struct cpu_ent {
 	timer			quantum_timer;
 
 	// keeping track of CPU activity
+	seqlock			active_time_lock;
 	bigtime_t		active_time;
 	bigtime_t		irq_time;
 	bigtime_t		interrupt_time;