x86 vdso gettc: Add RDTSCP support

Detect and use RDTSCP if available, instead of fence+RDTSC. For AMD Zens+,
use LFENCE+RDTSC instead of RDTSCP (or MFENCE;RDTSC previously).

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