gdb: adjust for external -> external/x64 move

No arch autodetection yet.

Reviewed-by: Claudio Fontana <claudio.fontana@huawei.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
gdb: fix 'osv info threads' for std::chrono::time_point<> change

The representation of time has changed; update 'osv info threads' so it
works again.

Reviewed-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>