tsc: add RDTSCP or faster variants of get_timecount()

Use it in preference of Xfenced RDTSC if RDTSCP is supported. It is
recommended by both Intel and AMD. But, on AMD Zens and newer use
LFENCE, as recommended by AMD [*]. In particular, this means that now
AMD CPUs use more appropriate fence instead of too harsh MFENCe.

Add comment explaining the intent of the selection logic.

Reported by: gallatin [*]
Reviewed by: gallatin, markj
Tested by: gallatin, pho
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D27986