gdb: report specific stop reason for watchpoints

The remote protocol allows for implementations to report more specific
reasons for the break in execution back to the client [1]. This is
entirely optional, so it is only implemented for amd64, arm64, and i386
at the moment.

[1] https://sourceware.org/gdb/current/onlinedocs/gdb/Stop-Reply-Packets.html

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

(cherry picked from commit 7446b0888d920124516284eaa32714d63353e2c9)