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.

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

(cherry picked from commit 0ef474de88fdb78e3cb7ac9bc0b1b98e5b49b4e4)