x86: Fall back to leaf 0x16 if TSC frequency is obtained by CPUID and
leaf 0x15 is not functional.

This should improve automatic TSC frequency determination on
Skylake/Kabylake/... families, where 0x15 exists but does not provide
all necessary information. SDM contains relatively strong wording
against such uses of 0x16, but Intel does not give us any other way to
obtain the frequency. Linux did the same in the commit
604dc9170f2435d27da5039a3efd757dceadc684.

Based on submission by: Neel Chauhan <neel@neelc.org>
PR: 240475
Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D21777