x86: fix dump_registers

We are usign %x instead of %lx for x86_64. This has the effect of trimming all
64 bits to 32 bits creating a very, very confusing situation for people
debugging OSv.

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