Support guest rdtscp and rdpid instructions on Intel VT-x

Enable any of rdtscp and/or rdpid for bhyve guests on Intel-based hosts
that support the "enable RDTSCP" VM-execution control.

Submitted by: adam_fenn.io
Reported by: chuck
Reviewed by: chuck, grehan, jhb
Approved by: jhb (bhyve), grehan
MFC after: 3 weeks
Relnotes: Yes
Differential Revision: https://reviews.freebsd.org/D26003