i386: provide PCPU pc_small_core for amd64 compat

Provide pc_small_core for i386 too to fix an i386 build break from x86
code referring to it. It won't be set.

Reviewed by: aokblast, kib
Fixes: 7b26353a59d6 ("hwpstate_intel: Disable package control on hybrid CPU")
Differential Revision: https://reviews.freebsd.org/D58335

(cherry picked from commit 29d15d658d175139196d821b123c30a5b58e135e)