Make PCB structure binary compatible for old and new PMAP on ARM

This structure must be binary compatible regardless of PMAP
version being used. Create reserved section for NEW_PMAP to
make other variables be placed exactly in the same memory
addresses. This fixes kgdb/gdb behavoiur, which uses pcb.h stuctures.
The NEW_PMAP is kernel flag, so it does not propagate to the buildworld,
what makes the tools using pcb.h unable to parse PCB data.

Reviewed by: mmel, kib
Submitted by: Wojciech Macek <wma@semihalf.com>
Obtained from: Semihalf
Sponsored by: Juniper Networks Inc.
Differential Revision: https://reviews.freebsd.org/D4011