gdb(4): handle single register read packets

We support bulk reads of the register set, but not reading specific
registers via the 'p' packet. This is useful at least for the 'call'
command in gdb.

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

(cherry picked from commit fd29833d9a7d8355ce21743ed8c706b72ba03363)