Implement get_pcpu() for i386 and use it to replace pcpu_find(curcpu)
in the i386 pmap.

The curcpu macro loads the per-cpu data pointer as its first step,
so the remaining steps of pcpu_find(curcpu) are circular.

get_pcpu() is already implemented for arm, arm64, and risc-v.
My plan is to implement it for the remaining architectures and use
it to replace several instances of pcpu_find(curcpu) in MI code.

Reviewed by: kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D9370