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.

Noted by: skra
Reviewed by: skra
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D9312