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
Sponsored by: NetApp, Inc.
Sponsored by: Klara, Inc.
NetApp PR: 51

(cherry picked from commit 4beb385813c8b1014f8250a31b07fdc09a059713)