truss: fix handling of 64-bit arguments/return values for compat32

Deciding whether to combine two values to a 64-bit one should be based on
the process ABI, and not dependent on whether truss is compiled for an
LP64 ABI. This is a follow-up cleanup for D27625. I found this while
looking for uses of the __LP64__ macro (since using this is wrong for
CHERI systems).

Test Plan: truss still works. Since I tested on AMD64 and all syscalls
in the trace have their quad argument last there was no difference
in the output. Should fix output for compat32 on MIPS64 though.

Reviewed By: jhb
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D27637

(cherry picked from commit 8ba2e89e987c7ae171c5f099e59d599294d626b7)