MFC r312610, r312792

r312610:
Like r310481 for i386, move the objects used to create temporary
mappings for armv6 pmap zero and copy operations to the MD PCPU region.
Change sysmap initialization to only allocate KVA pages for CPUs that
are actually present.

While here, collapse CMAP3 into CMAP2 (their use was mutually exclusive
anyway) and "recover" some space in PCPU padding that has always been
available due to 64-byte cacheline padding.

r312792:
Further cleanup of per-CPU armv6 pmap data:

• Replace pcpu_find(curcpu) with get_pcpu(), which is much more direct.
• Remove armv4 pcpu fields which I added in r286296 but never needed to use.
• armv6 pc_qmap_addr was leftover from the old armv6 pmap implementation. Rename it and put it to use in the new one.