amd64: allow gdb(4) to write to most registers

Similar to the recent patch to arm's gdb stub in r368414, allow GDB to
update the contents of most general purpose registers.

Reviewed by: cem, jhb, markj
MFC after: 2 weeks
Sponsored by: NetApp, Inc.
Sponsored by: Klara, Inc.
NetApp PR: 44
Differential Revision: https://reviews.freebsd.org/D27642