There is no universal way to find the TSC frequency.  Newer Intel CPUs
may report it via CPUID leaves 0x15 and 0x16.  Sometimes it can be
obtained from the PLATFORM_INFO MSR as well, though we never use that.
On older platforms we derive the frequency using a DELAY(1000000) call,
which uses the 8254 PIT.  On some newer platforms the 8254 is apparently
non-functional, leading to bogus calibration results.  On such platforms
the TSC frequency must be available from CPUID.  It is also possible to
disable calibration with a tunable, in which case we try to parse the
brand string if the TSC freq is not available from CPUID.

CPUID 0x15 provides an authoritative TSC frequency value, but even that
is not always available on new Intel platforms.  CPUID 0x16 provides the
specified processor base frequency, which is not the same as the TSC
frequency.  Empirically, it is close enough for early boot, but too far
off for timekeeping: on a Comet Lake NUC, CPUID 0x16 yields 1600MHz but
the TSC frequency is rougly 1608MHz, leading to frequent clock stepping
when NTP is in use.

Thus we have a situation where we cannot calibrate using the PIT and
cannot obtain a precise frequency from CPUID (or MSRs).  This change
seeks to address that by using the CPUID 0x16 value during early boot
and refining the calibration later once ACPI-based timecounters are
available.  TSC frequency detection is thus split into two phases:

Early phase:
- On Intel platforms, query CPUID 0x15 and 0x16 and use that value
  initially if available.
- Otherwise, get an estimate using the PIT, reducing the delay loop to
  100ms from 1s.
- Continue to register the TSC as the CPU ticks provider early, even
  though the frequency may be off.  Otherwise any code executed during
  boot that uses cpu_ticks() (e.g., context switching) gets tripped up
  when the ticks provider changes.

Later phase:
- In SI_SUB_CLOCKS, once the timehands are initialized, load the current
  TSC and timecounter (sbinuptime()) values at the beginning and end of
  a 1s interval and use the timecounter frequency (typically from
  kvmclock, HPET or the ACPI PM timer) to estimate the TSC frequency.
- Update the TSC timecounter, global tsc_freq and CPU ticker with the
  new frequency and finally register the TSC as a timecounter.

Reviewed by:	kib, jhb (previous version)
Discussed with:	imp, cperciva
MFC after:	6 weeks
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D32512