Partially revert r219672. After r198295, kernel need to seed randomness as

soon as possible for stack protector.  However, dummy timecounter does not
have enough entropy and we don't need to sacrifice Pentium class and later.

Pointed out by:	Maxim Dounin (mdounin at mdounin dot ru)

sys/i386/include/cpu.h

@@ -69,10 +69,14 @@ void	swi_vm(void *);
 static __inline uint64_t
 get_cyclecount(void)
 {
+#if defined(I486_CPU) || defined(KLD_MODULE)
 	struct bintime bt;
 
 	binuptime(&bt);
 	return ((uint64_t)bt.sec << 56 | bt.frac >> 8);
+#else
+	return (rdtsc());
+#endif
 }
 
 #endif