Move the objects used to create temporary mappings for i386 pmap zero and copy
operations to the MD PCPU region. Change sysmap initialization to only
allocate KVA pages for CPUs that are actually present. As a minor
optimization, this also prevents false sharing between adjacent sysmap objects
since the pcpu struct is already cacheline-aligned.

While here, move pc_qmap_addr initialization for the BSP into
pmap_bootstrap(), which allows use of pmap_quick* functions during early boot.

Reviewed by: kib
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D8833