libdtrace: Generalize handling of data models a bit
Make it easier to support data models other than ILP32 and LP64 by
avoiding constructs which assume that it must be one or the other.
No functional change intended.
MFC after: 2 weeks
Sponsored by: Innovate UK
(cherry picked from commit 096a5c6cd28c417456d5ce3598be15e6b656af5c)