kernel: Final spin in !MULTITHREADING should be locked

Now that we call main() with interrupts enabled in !MULTITHREADING, we
need to disable them again for the final fallback "loop-forever
because user code returned" state.  Otherwise some architectures will
toss interrupts into a context where we obviously aren't prepared.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>