cputimer: Allow MP synchronized TSC to become cputimer on x86_64

sysclock_t stays as 32bits, as suggested by dillon@.

The TSC cputimer frequency is scaled down TSC frequency (less or equal
to 128Mhz), which makes 32bits sysclock_t counter hold at least 33
seconds.

The TSC cputimer is obviously faster than other cputimers, e.g. HPET or
ACPI timer, so it has the highest priority as of this commit.  It is
enabled by default, if MP synchronized TSC is detected.  It could be
disabled by setting tunable hw.tsc_cputimer_enable to 0.