tls: Introduce struct dtv and struct dtv_slot

Rather than treating the DTV as a raw array of uintptr_t, use proper
struct types and gain the benefit of having different types for
different members. In particular, the module slots now have real pointer
types so less casting is generally needed.

Note that, whilst struct dtv_slot may seem a little unnecessary, this
will help downstream in CheriBSD where we wish to be able to easily
alter the layout of a module's slot, which this helps abstract.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D50231