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

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