x86 vdso gettc: reorganize ifunctions.
Instead of providing ifuncs for each kind of fence, define ifuncs
that combine fence and invocation of RDTSC. This refactoring makes
introduction of RDTSCP use possible.
Reviewed by: gallatin, markj
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D27986