debug: Add format specifiers to debug_ll()

Needed to make dump_registers() safe to call from fault handler.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
x64: Make dump_register() fault-handler safe

The debug() call can deadlock because it's using boost format. Switch to
debug_ll().

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