stand: use archsw.arch_copyin instead of i386_copyin

Since archsw.arch_copyin is always i386_copyin, this will be a nop in
terms of functionality. This is a diff reduction against other copies of
the code that differ only by what copyin routine they call.

Sponsored by: Netflix
Reviewed by: tsoome
Differential Revision: https://reviews.freebsd.org/D36568

(cherry picked from commit a705c72f214293a7aae5f83235a82f126f73e109)