Reimplement the ordering requirements for the timehands updates, and
for timehands consumers, by using fences.

Ensure that the timehands->th_generation reset to zero is visible
before the data update is visible [*]. tc_setget() allowed data update
writes to become visible before generation (but not on TSO
architectures).

Remove tc_setgen(), tc_getgen() helpers, use atomics inline [**].

Noted by: alc [*]
Requested by: bde [**]
Reviewed by: alc, bde
Sponsored by: The FreeBSD Foundation
MFC after: 3 weeks