gdb: allow setting/removing hardware watchpoints

Handle the 'z' and 'Z' remote packets for manipulating hardware
watchpoints.

This could be expanded quite easily to support hardware or software
breakpoints as well.

https://sourceware.org/gdb/onlinedocs/gdb/Packets.html

Reviewed by: cem, markj
MFC after: 3 weeks
Sponsored by: NetApp, Inc.
Sponsored by: Klara, Inc.
NetApp PR: 51
Differential Revision: https://reviews.freebsd.org/D29173