truss: split counting of syscalls and syscall calling convention

This change is a refactoring cleanup to improve support for compat32
syscalls (and compat64 on CHERI systems). Each process ABI now has it's
own struct sycall instead of using one global list. The list of all
syscalls is replaced with a list of seen syscalls. Looking up the syscall
argument passing convention now interates over the fixed-size array instead
of using a link-list that's populated on startup so we no longer need the
init_syscall() function.
The actual functional changes are in D27625.

Reviewed By: jhb
Differential Revision: https://reviews.freebsd.org/D27636