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.

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