Add support for setting hardware breakpoints from ptrace on arm64.

Implement get/fill_dbregs on arm64. This is used by ptrace with the
PT_GETDBREGS and PT_SETDBREGS requests. It allows userspace to set hardware
breakpoints.

The struct dbreg is based on Linux to ease adding hardware breakpoint
support to debuggers.

Reviewed by: jhb
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D22195