truss: centralize pointer-constructing casts.

In nearly all cases, the caller has a uintptr_t compatible argument so
this eliminates a large number of casts.

Add a print_pointer function to centralize printing pointers.

Reviewed by: jhb
Obtained from: CheriBSD
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D22212