x64: Make DUMMY_HANDLER less dumb

It's nice to at least see the RIP where the invalid opcode exception
happened at...

Reviewed-by: Glauber Costa <glommer@cloudius-systems.com>
Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>