setcred: Move initial copyin of struct setcred out to per-ABI syscall

This is the more typical approach used in the tree for system calls
with per-ABI structure layouts.

Reviewed by: olce, brooks
Obtained from: CheriBSD
Sponsored by: AFRL, DARPA
Differential Revision: https://reviews.freebsd.org/D53756

(cherry picked from commit 6292eecfd95c78abc1ab14f20ceaa507ab9c636a)