gdb(4): allow bulk write of registers

Add support for the remote 'G' packet. This is not widely used by gdb
when 'P' is supported, but is technically required by any remote gdb
stub implementation [1].

[1] https://sourceware.org/gdb/current/onlinedocs/gdb/Overview.html

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

(cherry picked from commit 3f3cc995a35a3e9136204a98af0af5808c11047f)