sys/ddb: Add hardware breakpoint support to ddb

As with hardware watchpoints add support for hardware breakpoints. The
command is only enabled on architectures that report support for them.
Currently no architectures do, however arm64 will add support in a
future change.

Reviewed by: jhb (earlier version)
Sponsored by: Arm Ltd
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D22191