In order to do that cleanly, lapic_setup_clock(), on both ia32 and amd64,
now accepts as arguments the desired sources to handle, and returns the
actual ones (LAPIC_CLOCK_NONE is forbidden because otherwise there is no
meaning in calling such function).
This allows to bring out into commont x86 code the handling part for
machdep.lapic_allclocks tunable, which is retained.