x64: fix early halt()

When halt() is called very early, before smp_launch(), it crashes when
calling crash_other_processors() because the other processors' IDT was
not yet set up. For example, in loader.cc's prepare_commands() we call
abort() when we failed to parse the command line, and this caused a
crash reported in issue #252.

With this patch, crash_other_processors does nothing when other processors
have not yet been set up. This is normally the case before smp_launch(),
but note that on single-vcpu VM, it will remain the case throughout the run.

Fixes #252.

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