stand: use archsw.arch_copyin instead of direct call

This replaces the CALLBACK(copyin, ...) with a call to
archsw.arch_copyin which points to a function that does the
callback. More diff reduction for the multiple copies of these routines
in the tree.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D36569

(cherry picked from commit 4c670b53a0007739a0fc84ea4bc24a5895953aae)