Use cpu_ticks() for get_cyclecount(9) rather than checking existence of TSC
at run-time on i386. cpu_ticks() is set to use RDTSC early enough on i386
where it is available. Otherwise, cpu_ticks() is driven by the current
timecounter hardware as binuptime(9) does. This also avoids unnecessary
namespace pollution from <machine/cputypes.h>.