get_pcpu() already exists for arm/arm64/risc-v
as a more direct alternative to pcpu_find(curcpu).
The curcpu macro already loads the per-cpu data
pointer as its first step, so the following steps
of loading the cpuid field and using that to index
the per-cpu data array via pcpu_find() are circular.

Replace pcpu_find(curcpu) with get_pcpu() in i386
pmap. My plan is to implement get_pcpu() for the
remaining architectures and use it to replace
instances of pcpu_find(curcpu) in MI code.