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

Reviewed by: cem
MFC after: 1 week
Sponsored by: NetApp, Inc.
Sponsored by: Klara, Inc.
NetApp PR: 44
Differential Revision: https://reviews.freebsd.org/D27644